The periderm forms a protective layer around the outside of many stems and roots and consists of cork cambium, cork, and phelloderm. Cork cambium is the site of active secondary growth within the periderm of vascular plants. Phloem is a type of vascular tissue that directs the flow of nutrients and metabolic products from the leaves down to the roots.

As described in the beginning of this question, collenchyma cells are found in young stems and petioles (leaves) and function to provide flexible support to the plant. This is because chollenchyma cells lack secondary cell walls and do not produce lignin to harden them—this protein is characteristic of sclerenchyma cells, which are also used to provide support/strength to the plant. 

Due to their lack of rigidity, collenchyma cells a also capable of elongating with the stems and leaves they support, allowing them to remain alive at maturity. Sclerenchymal cells lack this ability.

As described in the background to the question, sclerenchyma cells are specialized to support the plant as it grows. These cells have thick secondary walls that are further strengthened by the hardening agent called lignin.  As a result, these cells are highly rigid and inflexible.  

At maturity, these cells cannot elongate and are found in regions of the plant that have stopped growing. In some parts of the plant, the sclerenchyma cells may even be dead; however, the rigid walls remain and act like a skeleteon that supports the remainder of the plaint over its lifetime.  

Sclerenchyma cells can also further differentiate into two types called sclereids and fibers. Sclerids can provide hardness to nut shells. Fibers, as their name suggests, are usually arranged in long threads and have commercial uses, such as being made into rope.

For proper functioning, plants must take in carbon dioxide, expel oxygen, and limit the loss of water vapor. This gas exchange takes place via pores called stomata. These pores are formed by a pair of adjacent cells that can open and close in response to a number of factors. These cells are called guard cells.

The cuticle and epidermis are layers of leaf structure, and do not correspond to specific cell types. The stoma is the name of a single pore itself, not its surrounding cells.

Cellulose, a polymer of glucose, is the main component of plant cell walls. 

Collagen is found in the connective tissues of animals. Chitin is found in the cell walls of fungi. Actin and myosin are the proteins responsible for contraction in muscle cells; actin is also a microfilament in the cytoskeleton. Peptidoglycan is found in the cell walls of bacteria.

The correct answer is aquaporins. While water can move across a membrane via simple diffusion, these transmembrane proteins increase the flow of water. Remember that water is a polar molecule, and is thus relatively impermeable to the plasma membrane despite its small size.

Angiosperms are flowering plants. They have both male and female reproductive organs.

The pistil is the female reproductive structure, and consists of three parts. The stigma is the sticky knob used to catch pollen. It is situated at the top of the style, while the ovary contains egg cells and is located below the style. 

The stamen is the male reproductive structure, and consists of two parts. The anther produces pollen, and the filament helps in holding up the anther.

Double fertilization is the process by which two sperm cells are introduced to the ovule. One sperm () fertilizes the egg (), creating a zygote(). The other sperm combines with the two polar nuclei (), forming the endosperm () that will nourish the embryo.

The correct answer is auxin. IAA is the most abundant auxin naturally found in plants, and plays key roles in plant development and growth. 

Ethylene gas promotes fruit ripening; in a classic example of a positive feedback loop, ripe fruit causes the production of more ethylene gas, which promotes more fruit ripening (this is the origin of the phrase "one bad apple spoils the bunch!")

Cytokinins stimulate cell division and cytokinesis, and they can delay senescence (deterioration due to age, as in the case of cells). Gibberellins promote cell growth, and also promote stem and leaf elongation. Abscisic acid (ABA) promotes seed dormancy, inhibits plant growth, and closes plant stomata during water shortages.