Final answer:
Sperm and egg cell maturity commences at puberty, which is when reproductive capability begins for both males and females.
Step-by-step explanation:
The maturity of sperm and egg cells begins at puberty. This is the phase of life when the body undergoes various changes as individuals transition from childhood to adulthood, marking the start of reproductive capability.
In terms of oogenesis, female ovaries contain all the eggs she will ever produce at birth. These eggs remain immature until puberty. Once puberty begins, hormone-triggered changes allow one egg to typically mature each month. This process continues monthly until a woman approaches menopause, which marks the end of her reproductive years when fertility declines and eventually ceases. In contrast, spermatogenesis, the production of sperm in males, begins at puberty as well and continues throughout most of a man's life.
Therefore, contrary to option 'a', which suggests maturity begins at conception, or 'd', indicating it begins at menopause for women, it is during puberty that both males and females become capable of producing mature reproductive cells.