What is plasma primarily composed of?

Plasma is primarily composed of water and proteins, making this the correct answer. Water constitutes about 90-92% of plasma volume, serving as the primary solvent for carrying various substances throughout the body. The proteins present in plasma, such as albumin, globulins, and fibrinogen, play crucial roles in maintaining osmotic pressure, transporting hormones and nutrients, and facilitating blood clotting. This composition is essential for various physiological functions, including transporting nutrients, waste products, and maintaining blood volume and pressure.

The other choices reflect components that are indeed part of blood but do not accurately describe plasma's primary composition. For example, red blood cells, platelets, and white blood cells are cellular components of blood that are suspended within the plasma but are not part of its primary makeup. Fluids and nutrients are aspects of what plasma carries, not its primary constituents.