Aja is worried that as her husband approaches his 40th birthday he will become depressed and unhappy with his life, and that he will begin to seek out adventure and younger women to help compensate for these feelings. Studies indicate that very few men (or women) go through this sort of:__

Midlife crisis.

Explanation:

As the exercise suggests in the example, what Aja fears her husband will be engaged in is in a midlife crisis. This is usually defined as feelings of anxiety or disappointment in their lives that, perhaps, is found out at that age. Therefore, Aja fears that her husband will encounter those feelings and will begin to compensate with "adventurous" things like seeing younger women.