Subnetting Question

I was recently asked a question in a quiz and as easy as it looks, i get a
bit confused.

An organization is granted a block; one address is 2.2.2.64/20. The
organization needs 10 subnets. What is the subnet prefix length?

a) /20
b) /24
c) /25
d) None of the above

Answers would be greatly appreiated