Which of these is not one of the three blocks RFC 1918 reserves for private use?
Answer
172.32.0.0/12
The private class B range runs 172.16.0.0 through 172.31.255.255, which is what the /12 prefix covers. 172.32 is one step past the end of it and is public address space, which is why picking an address there by accident produces a network that appears to work locally and quietly breaks when someone tries to reach the real owner of that block. The other three are the reserved blocks exactly as written, one carved out of each of the old classes A, B, and C.