Fluid intelligence peaks in the twenties and declines from there; crystallized intelligence keeps rising into old age. Why does the distinction predict that an older adult can be worse at a novel puzzle and better at their job?

C. Because the two draw on different resources.

The two abilities have different inputs, which is why they have different trajectories: fluid reasoning has no prior knowledge to draw on, and crystallized ability is the accumulated knowledge. An older engineer solving a problem in her field is deploying pattern recognition built over decades, put her in front of a puzzle designed to defeat prior knowledge and the support is removed.