People are mad . . . . . money

The correct answer is B: after.
Here's why:
"Mad after" is an idiom that means to be very enthusiastic about or to desire something greatly.
In this context, "People are mad after money" means that people strongly desire or are very enthusiastic about getting money.
Let's look at why the other options are incorrect:
A: over: "Mad over" usually implies anger or frustration about something. It doesn't fit the context of desiring money.
C: in: "Mad in" is not a common or correct phrase in this context.
D: with: "Mad with" usually implies being driven to madness by something, like "mad with grief." It doesn't fit the context of desiring money.
Therefore, after is the only option that creates a meaningful and grammatically correct sentence, describing a strong desire for money.