During which age range do most girls' bodies begin to undergo changes such as breast development and hair growth under the arms?

5-9
8-14
15-18
19-25

The correct answer is 8-14.

Nearly all the organ systems in the human body starts to grow and develop after birth by growing their different systems. But the reproductive system remains dormant for many years due to secretion of very small amount of gonadotropins (reproductive hormones) from the hypothalamus of human brain. Less secretion of gonadotropins checks the sexual development in girls and boys. On attaining the age about 8-14 in girls, there is sudden increase in secretion of gonadotropins from the brain. This activates the development of secondary sexual characters such as breast development and hair growth under the arms. This is onset of puberty beginning of adult sexual life.