Oxygenated blood returns to the left atrium through which of the following?

A. The plasma membrane, also known as the cell membrane, surrounds the entire cell and separates the cytoplasm from the external environment. It acts as a barrier, controlling the movement of substances into and out of the cell.

B. The Golgi apparatus is involved in modifying, sorting, and packaging proteins and lipids, but it does not enclose the cytoplasm. It is an internal organelle.

C. The endoplasmic reticulum (ER) is involved in protein and lipid synthesis and forms a network within the cytoplasm, but it does not enclose it. It works within the cell's internal structure.

D. The flagellum is a tail-like structure used for cell movement in some cells, but it does not enclose the cytoplasm.

A. Mast cells do not function to connect the skin to muscles. This is the role of connective tissues such as tendons.

B. Mast cells do not cushion bony prominences. Adipose tissue and connective tissue help to provide cushioning.

C. Mast cells are involved in the inflammatory response. They release histamine and leukotrienes, which play a significant role in initiating inflammation, especially during allergic reactions and immune responses. This is the correct answer.

D. While mast cells are involved in immune responses, their main role is not to directly destroy pathogens. Other cells like macrophages and neutrophils are responsible for pathogen destruction.