What are the three kinds of blood cells?

The three main kinds of blood cells are erythrocytes, thrombocytes, and leukocytes. Erythrocytes, also known as red blood cells, are responsible for transporting oxygen from the lungs to the body's tissues and facilitating the return of carbon dioxide from the tissues back to the lungs. Thrombocytes, commonly known as platelets, play a crucial role in blood clotting and maintaining hemostasis, which is essential for preventing excessive bleeding. Leukocytes, or white blood cells, are key components of the immune system; they help the body fight infections and perform various immune functions.

Other choices listed, such as plasma, serum, and white blood cells, do not accurately reflect the three primary types of blood cells. While plasma is the liquid component of blood that carries these cells, it is not considered a type of blood cell. Similarly, while "white blood cells" refers to leukocytes, the options misrepresent the complete set by either including non-cell components or using incorrect terminology. Thus, the correct answer highlights the essential cellular components that make up the blood, directly linking to their respective functions within the body’s circulatory and immune systems.