What are the primary components of blood?

The primary components of blood are indeed red blood cells, white blood cells, platelets, and plasma.

Red blood cells, or erythrocytes, are responsible for the transport of oxygen from the lungs to the rest of the body and carbon dioxide back to the lungs for exhalation. White blood cells, or leukocytes, play a critical role in the immune system, helping to defend against infections and foreign substances. Platelets, or thrombocytes, are vital for blood clotting and help prevent excessive bleeding when injuries occur. Plasma, the liquid component of blood, serves as a medium that carries all of these cells along with nutrients, hormones, waste products, and proteins.

While other options include related substances, they do not represent the fundamental components of blood comprehensively. For instance, serum is a component found in blood but is not a primary element like the four chosen in the correct answer. Similarly, hormones and electrolytes are carried in the blood but are not classified as primary cellular components. Hence, option B accurately encapsulates the core composition of blood that plays essential and distinct roles in maintaining overall health and body function.