Which substance in the blood is important for regulating body temperature and delivering nutrients?

Plasma is the correct choice because it serves several key functions in the blood, one of which is regulating body temperature. Plasma is the liquid component of blood, making up about 55% of its volume. It consists mostly of water, and has a high heat capacity, which allows it to absorb and distribute heat throughout the body. This helps maintain a stable internal temperature, which is crucial for the proper functioning of various biological processes.

In addition to temperature regulation, plasma plays a vital role in delivering nutrients to cells. It transports essential substances, including glucose, amino acids, vitamins, and minerals, which are dissolved in the liquid. This function is critical for cellular metabolism and overall health. Additionally, plasma carries waste products away from cells to be excreted from the body, contributing to homeostasis.

While other components of blood, such as red blood cells and white blood cells, have important roles—such as transporting oxygen and fighting infections, respectively—they do not specifically regulate body temperature or directly deliver nutrients like plasma does. Proteins in the blood, including albumin and globulins, are important for various functions, but they are primarily aspects of the plasma and do not function independently in the context described in the question.