Final answer:
Primary sex characteristics are the organs necessary for reproduction, such as the uterus in females and the testes in males, which develop and mature during adolescence. Therefore correct option is D
Step-by-step explanation:
The true statement about the primary sex characteristics of humans is D. They are the organs necessary for reproduction. Primary sex characteristics refer to the biological organs and structures that are directly involved in reproduction, such as the uterus and ovaries in females, and the testes in males. These characteristics usually develop and mature at adolescence, in contrast to secondary sex characteristics like facial hair in males and the development of breasts in females, which are not directly involved in the reproductive process.