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.