Which of the following substances is blood primarily composed of?

Blood is primarily composed of plasma, red blood cells, white blood cells, and platelets. Plasma constitutes about 55% of blood volume and serves as the liquid medium that carries cells, nutrients, hormones, and waste products. Red blood cells are responsible for transporting oxygen from the lungs to the rest of the body and bringing carbon dioxide back to the lungs for exhalation. White blood cells play a crucial role in the immune system, defending the body against infections and diseases. Platelets are vital for blood clotting, preventing excessive bleeding when injuries occur.

Each of the other choices lists components or substances that might be found within blood but do not accurately represent its primary composition. For example, while oxygen and nutrients are important for the body's functioning, they are not the main components that make up blood. Similarly, serum, hormones, and enzymes are indeed present in blood, but they do not capture the complete structural makeup of blood itself. Finally, although water, minerals, and vitamins are crucial for overall health, they do not represent the primary constituents of blood as a physiological fluid.