Final answer:
During middle adulthood, crystallized intelligence, which involves knowledge and skills obtained from life experience, remains stable or improves, enhancing abstract reasoning skills.
Step-by-step explanation:
The question asks which cognitive change occurs during middle adulthood. The correct answer is that abstract reasoning skills improve, corresponding with an increase in crystallized intelligence. Crystallized intelligence refers to the knowledge and skills that an individual acquires through lifetime experience, and typically this type of intelligence remains stable or can even improve during middle adulthood. On the other hand, fluid intelligence, which involves the ability to see complex relationships and solve problems, generally begins to decline in late adulthood.