What signifies the transition into puberty in humans?

The transition into puberty in humans is primarily signified by changes in body shape and appearance. This developmental stage involves a series of physiological and hormonal changes that lead to the maturation of the body. During puberty, the body undergoes significant transformation, including the development of secondary sexual characteristics such as breast development in females and increased muscle mass and body hair in males. These changes are driven by hormones like estrogen and testosterone, marking the onset of reproductive maturity.

While other options highlight important aspects of puberty, they do not encapsulate the broader physical changes that define this period. The increase in metabolic rate is a consequence of hormonal changes but not a defining feature. The development of primary reproductive cells is indeed a part of puberty but focuses more specifically on the reproductive system's maturation rather than the overall transition. Likewise, the initiation of menstruation is an important milestone, but it typically occurs later in puberty and does not represent the initial changes associated with this transformative phase. Therefore, the change in body shape and appearance serves as the most comprehensive indicator of entering puberty.