A general slowdown explains much of cognitive aging

Correct: B — A general slowing of processing speed accounts for much of the age-related decline observed across cognitive domains

Salthouse (1996) marshalled mediational evidence that age-related declines on reasoning, working memory, and episodic-memory tasks largely vanish once individual differences in basic processing speed are partialled out. His 'limited-time' and 'simultaneity' mechanisms argue that slower processing leaves earlier results unavailable when later operations need them. The theory remains the most influential domain-general account of cognitive aging, even as later work has clarified that processing speed is one mediator among several (working-memory capacity, inhibitory control, executive function) rather than the sole driver.