What is a major role of protein in the body?

The correct choice highlights a crucial role of proteins in the body, specifically maintaining water balance. Proteins, particularly those that are classified as transport proteins, play a significant part in regulating the movement of water and electrolytes across cell membranes. This function is essential for maintaining proper fluid balance in the body's tissues and blood, allowing for the optimal functioning of cells, organs, and systems.

In addition, proteins like albumin in the blood help to maintain osmotic pressure, which is vital for keeping fluids within the circulatory system. When protein levels drop, it can lead to issues such as edema, where excess fluid accumulates in tissues. Thus, maintaining water balance is pivotal for overall physiological stability.

Other roles of proteins include acting as enzymes, supporting immune functions, transporting nutrients, and providing structural support to cells and tissues. However, the maintenance of water balance specifically highlights the importance of proteins in fluid regulation, which is a fundamental aspect of homeostasis.