Blood is composed of only two main parts: plasma and _____.

Blood is composed of two main components: plasma and cells. Plasma is the liquid portion of blood that carries water, electrolytes, nutrients, hormones, and waste products. The cells in blood include red blood cells, which transport oxygen; white blood cells, which are integral to the immune system; and platelets, which are essential for blood clotting. This division into plasma and cells highlights the functional roles each component plays in maintaining homeostasis in the body.

The other choices—minerals, oxygen, and vitamins—do not represent the primary structural components of blood. While minerals and vitamins are important for health and support various functions in the body, they are not main components of blood itself. Oxygen is indeed a critical factor related to red blood cells, but it is not considered a structural component as blood is generally categorized. Therefore, the correct answer highlights the fundamental composition of blood.