Which component makes up the fluid portion of blood?

Plasma is the correct choice as it constitutes the fluid portion of blood. It is a yellowish fluid that makes up about 55% of total blood volume and is primarily composed of water, electrolytes, proteins, hormones, and waste products. Plasma serves essential functions, including transporting nutrients, gases, and waste products throughout the body.

Serum is a component that forms when blood clots and the liquid part is separated, which means it does not contain clotting factors. Hematocrit refers to the proportion of blood that is made up of red blood cells, expressed as a percentage. Hemoglobin, on the other hand, is a protein found within red blood cells that is responsible for transporting oxygen. Thus, while all these components are vital to blood function, plasma is specifically recognized as the fluid aspect that facilitates the transport of various substances.