Final answer:
Crystallized intelligence helps explain why a scientist is more productive in his 50s than in his 20s.
Step-by-step explanation:
The form of intelligence that helps explain why a scientist is likely to be more productive in his 50s than in his 20s is crystallized intelligence. Crystallized intelligence refers to acquired knowledge and the ability to retrieve it. As the scientist gains more experience and knowledge over time, their crystallized intelligence increases, allowing them to be more productive in their work.