Say we use the following notations:
t = total number of books
m = male buyers
f = female buyers
ma = average number of books bought by male buyers
fa = average number of books bought by female buyers
You get that t = m*ma + f*fa = 15 000 and you are looking for f.
1. tells us that m + f = 4 000. However, we do not have any additional info regarding the relationship between m and f. This is why 1 is insufficient.
2. means that ma = 3 and fa = 5, making t = 3m + 5f = 15 000. This doesn't help establish f either, since you don't know any further info about f and m.

Now, but both equations together and you have a system of two unknowns and two equations, easily solvable:
m + f = 4 000
3m + 5f = 15 000.

So C is indeed correct.

thanks a lot!