Which of the following is a principal function of the circulatory system?

The circulatory system primarily functions to transport essential substances throughout the body, reinforcing the significance of nutrient distribution. This includes not only delivering glucose and amino acids obtained from digested food but also transporting vitamins and minerals needed for various cellular processes. By circulating these nutrients, the circulatory system supports cellular metabolism, growth, and repair.

The other options focus on functions that either involve different systems or are secondary functions of the circulatory system. For instance, the production of immune cells is primarily the role of the lymphatic system and certain organs like the bone marrow. While the circulatory system does assist in the elimination of wastes by transporting them to organs like the kidneys, the primary role is more aligned with nutrient and oxygen delivery. Similarly, absorption of oxygen takes place in the lungs during respiration and, while the circulatory system carries oxygen to the tissues, it is not the primary function itself. Thus, the correct answer emphasizes the circulatory system's essential role in distributing vital nutrients throughout the body.