Bacteria contain a cell wall made of peptidoglycan. This peptidoglycan is what is used when looking at the "Gram stain" of a bacteria. By inhibiting the formation of peptidoglycan, penicillin works by blocking the production of bacterial cell walls. Bacteria cannot survive without their cell walls.

The presence of chloroplasts indicate that Euglena is able to photosynthesize. These organelles contain pigments that are necessary for the conversion of light energy to chemical energy. The ability to use non-chemical energy (in this case light) to generate chemical energy is the defining characteristic of autotrophs.

The red eye spot, while it is involved in the complex of organelles that sense light, gives no information about trophic mode: many organisms sense and respond to light, but are not autotrophic.

All of these answers are true, except the statement that protists are definitionally unicellular. Protists are an incredibly diverse group of organisms that are difficult to classify because of the vast differences that exist between protist species; it is easier to classify protists based on what they are not, rather than what they are. Multicellular protists include species of seaweeds called kelp. Though kelp resemble plants, they are actually multicellular groups of protists that lack true stems, roots or leaves (though kelp often have structures resembling these plant structures).

Endosymbiosis is an intriguing theory that seeks to explain the presence of complex structures such as mitochondria and chloroplasts inside of plant and animal cells. Mitochondria's similarities to free-living proteobacteria, and chloroplasts' similarities to free-living cyanobacteria suggest that at one point these structures were organisms that became engulfed by larger cells. According to the theory of endosymbiosis, mitochondria and chloroplasts lived inside their hosts in a symbiotic relationship, eventually evolving to lose their capacity to live independently and becoming organelles that support their host cell.

Though all of these answers are true of at least some protists, the most generally applicable statement is that protists generally prefer to live in environments that contain plenty of water. Species within the kingdom Protista are believed to have given rise to the other kingdoms of Eukarya (these other kingdoms being Fungi, Animals and Land Plants), so the incredible diversity found within Protista is not particularly surprising. 

For the most part, protists are very valuable to humans. They form the base of a large portion of aquatic food chains, and without them many ecosystems would collapse. However, some protist species have caused—and continue to cause—profound problems to humans. Of the given answers, the only one not caused by a species of protist is AIDS. AIDS is caused by a retrovirus, which means that it is an RNA virus that uses reverse transcription to replicate itself in a host cell. 

Phytoplankton are the collection photosynthetic organisms (eukaryotic and prokaryotic) that are the primary producers that form the basis of most marine food chains. Zooplankton, fish, and even large whales feed on phytoplankton. Since phytoplankton are photoautotrophs, they need sunlight to perform photosynthesis, thus they are not found in the dark depths of the ocean.

Diatoms are unicellular phytoplankton that are incased in silica cell walls. These silica shells fall to the bottom of the ocean. Dinoflagellates are the bioluminescent phytoplankton with two flagella and are responsible for red tides. Diatoms have clear silica (glass) cell walls to allow sunlight into them, where they contain different pigments that absorb sunlight at various wavelengths. Note that although diatoms may form colonies, they are unicellular.

Budding is a form of asexual reproduction in which a new organism grows as a bud on the parent, and separates once it is fully mature. As a result, daughter cells have the same genetic information as their parent cell, but are not the same size.

While certain types of animals and bacteria can reproduce by budding, it is more common in fungi. Bacteria more commonly reproduce by binary fission, while animals usually incorporate sexual reproduction. Protista also commonly reproduce via budding, but are not given as an answer choice.

Like animals, fungi are chemoheterotrophs. They must get both their energy and carbon skeletons by absorbing pre-digested nutrients from the environment.

Heterotrophs are unable to create organic compounds without receiving an input of organic material from an outside source. In contrast, autotrophs are capable for generating organic compounds from inorganic compounds.

Chemoheterotrophs gain organic input by consuming other organic material. Fungi break down organic matter from the soil to generate chemical energy.

Lithoautotrophs use inorganic compounds to generate organic compounds, and photoautotrophs use light. Mixotrophs can use either organic or inorganic materials to generate energy.

Fungal cells, as well as plant and bacterial cells, are encased by a cell wall around their cell membrane. In fungi, this wall is composed of the polysaccharide chitin, the same hard substance that forms the exoskeletons of arthropods. Plants, on the other hand, have cell walls made of cellulose, while bacteria have peptidoglycan cell walls.

All of these cell types still have an inner cell membrane, which is composed of phospholipids.