Final answer:
The most likely physical change to begin in the 30s is the decline in sensitivity to high-frequency sounds, as the aging process commonly affects sensory abilities. Menopause typically occurs around age 50, and the increase in estrogen production is not characteristic in aging, with levels usually decreasing.
Step-by-step explanation:
Among the physical changes occurring in adults that are likely to begin in the 30s, the decline in sensitivity to high-frequency sounds is the most common. This is because aging often affects our sensory abilities, and it is well documented that our hearing, especially for high-frequency sounds, starts to diminish as we get older. During the 30s, many individuals begin to notice a slight decrease in their ability to hear higher pitches. This is a gradual process that continues into later adulthood.
Other options such as erectile dysfunction may occur, but typically begin later, and are not as commonly expected to start as early as the 30s. The increase in the production of estrogen is not a characteristic change of aging; in fact, levels usually decrease with age. Menopause generally occurs around the age of 50, not in the 30s. Therefore, among the given options, the decline in sensitivity to high-frequency sounds is the most likely to begin in the 30s.