Final answer:
The correct answer is C. Gradual; sustained, as Schaie's Seattle Longitudinal Study showed a gradual decline in certain mental abilities, with sustained modest gains in others during midlife.
Step-by-step explanation:
Findings of Schaie's Seattle Longitudinal Study revealed a gradual drop in five mental abilities after the mid-thirties, but modest sustained gains for those same abilities in midlife. The correct answer to the given question is C. Gradual; sustained. Schaie's study indicated that while certain cognitive abilities might start to decline as we reach our mid-thirties, the accumulation of life experience and knowledge can result in the sustained improvement of other abilities, such as crystallized intelligence.