Final Answer:
The onset of sexual maturity typically occurs earliest in a boy's puberty. This is because the hormonal changes leading to sexual maturity usually begin before other signs of puberty, such as facial hair growth, deepening of the voice, or a growth spurt in height. Therefore, the correct answer is d) Onset of sexual maturity.
Step-by-step explanation:
The onset of sexual maturity involves the development of the reproductive system, including the maturation of the testes and the production of sperm. This process, known as spermatogenesis, typically begins around the age of 12 or 13. In contrast, facial hair growth (a), deepening of the voice (b), and a growth spurt in height (c) are secondary sexual characteristics that occur as a result of increased levels of testosterone, which is produced during sexual maturity.
The growth spurt in height (c) usually occurs after the onset of sexual maturity. On average, boys experience a peak growth velocity around two years after the start of puberty. This is due to the growth plates in the bones closing, signaling the end of the growth phase. Facial hair growth (a) and deepening of the voice (b) typically occur later in puberty, usually after the initial stages of sexual maturity. These secondary sexual characteristics are influenced by the continued release of hormones and their effects on various tissues in the body.
In summary, while the exact timing can vary among individuals, the onset of sexual maturity (d) generally precedes the other listed events in a boy's puberty. The hormonal changes associated with sexual maturity initiate the development of primary sexual characteristics, setting the stage for the subsequent secondary sexual characteristics that follow. Therefore, the correct answer is d) Onset of sexual maturity.