Final answer:
Processing speed is a weak predictor of older adults' vocabulary knowledge, which is part of crystallized intelligence and tends to remain stable or improve with age, contrasting with the decline of fluid intelligence.
Step-by-step explanation:
Processing speed is a weak predictor of older adults' vocabulary knowledge. Unlike physical abilities that decline with age, crystallized intelligence consisting of accumulated knowledge and vocabulary tends to remain stable or may even improve with age. This refers to the skills, information, and strategies one has acquired throughout their life, and it contrasts with fluid intelligence, which encompasses processing speed, reasoning, and memory and tends to decline in later years.
Researchers have found that mental and physical activities can delay the onset of cognitive decline, suggesting that while processing speeds do decrease, the depth of vocabulary knowledge an older adult has is invariably strengthened via lifelong learning and experience. Additionally, other cognitive functions, such as the ability to organize information or the performance on certain neuropsychological tests, demonstrate that processing speed does not necessarily equate with other aspects of cognitive function like vocabulary knowledge.
Therefore, the correct answer to the question is D. Vocabulary knowledge.