The number of juries can be determined by using combinatorics. This will be nC12, with n the number of potential jurors.
1. What you have to notice here is that only 13C12 = 13 (this is because 13 is a prime number and therefore cannot be obtained by multiplying anything else). This means that n = 13 and m = 1, with 12 possible juries after eliminating that one guy. So 1 is sufficient.
2. is a bit easier to figure out. Since n = m + 12, this means that there are 12 people left to make up the jury after kicking that one guy out.
So answer is D.