Final answer:
Eric's development of underarm hair is an example of a secondary sex characteristic, which is a normal part of male puberty triggered by testosterone.
Step-by-step explanation:
Thirteen-year-old Eric has developed tufts of hair under his arms. This change is known as one of the secondary sex characteristics. Secondary sex characteristics are traits that are different in males and females but are not directly involved in reproduction. During puberty, males and females develop these characteristics, which signal sexual maturation. In males, testosterone stimulates the growth of characteristics such as pubic hair, underarm hair, facial hair, a deepened voice, and the beginnings of the sex drive. For Eric, the growth of hair under his arms is a normal part of this process and signifies that his body is proceeding through the stages of puberty.