Which of the following is a sign of sexual maturation in adolescent boys?

Testicular enlargement is a primary indicator of sexual maturation in adolescent boys, marking the onset of puberty. This change signals the body's preparation for the production of sperm and the increase in testosterone levels, which are crucial for developing secondary sexual characteristics. As boys transition through puberty, testicular enlargement typically occurs before other physical developments, such as increased body hair and growth spurts.

Menstruation is a sign of sexual maturation in females, not males, and breast development is also a characteristic change associated with female puberty. Underarm hair development does occur during male puberty, but it is a later sign compared to testicular enlargement, which is one of the earliest physical changes observed in boys as they mature sexually. Therefore, testicular enlargement serves as a significant milestone in the maturation process for adolescent boys.