Final answer:
During adulthood, fertility in both sexes does not remain stable; women experience a peak in fertility in their twenties before it declines, while men see a gradual decline in fertility starting around age 40.
Step-by-step explanation:
The question concerns the sexual-reproductive system and its impact on fertility during adulthood. Fertility refers to the natural capability to produce offspring. As adults, both men and women experience changes in fertility, but these changes do not usually lead to a 'stable fertility' for both sexes. For women, female fertility peaks in their twenties, then declines slowly until about age 35, after which it decreases more rapidly until menopause, when fertility ends. For men, while there is a gradual decline in male fertility starting around the age of 40, they generally retain some degree of fertility throughout most of their adult life. Therefore, the correct answer to the question would likely be that during adulthood, there is a general pattern of decreased fertility as one ages.