Let's analyse each option:
A.
Spending money means the money will decrease, so it would be represented by minus 32.
B.
Dropping a rock means its height will decrease, so minus 32 would fit better.
C.
The speed is decreasing, so minus 32 as well.
D.
The amount of books is increasing, so positive 32 fits better in this situation.
Therefore the correct option is D.
