Final answer:
The mental ability that declines steadily from one's twenties through the late eighties as identified in Schaie's Seattle Longitudinal Study is speed (C). Mental and physical activities can help delay the onset of cognitive decline, which is associated with fluid intelligence.
Step-by-step explanation:
According to Schaie's Seattle Longitudinal Study, the mental ability that decreases steadily from the twenties to the late eighties is speed(C). While crystallized intelligence, such as vocabulary, may remain stable or even improve with age, fluid intelligence which includes information processing abilities, reasoning, and memory, tends to decline. This decline encompasses the speed at which cognitive processes occur, leading to slower reactions and decision-making.
Engaging in both mental and physical activities has been found to help delay cognitive decline. These activities can reduce the incidence of mild cognitive impairment and dementia, suggesting that maintaining an active lifestyle is beneficial for preserving cognitive functions into later adulthood.