I say D

1) if one less potential juror had been rejected, it would be possibile to create 13 different juries

13 = 13!/12! Therefore we know that if 1 less juror has been rejected, there would have been 13 to choose from. 13-1 = 12
12!/12! = 1 possible jury
SUFFICIENT

2) n= m + 12

this statement tells us that all but 12 were rejected
m= n - 12 Therefore 12 were not rejected
12!/12! = 1
SUFFICIENT

What's the OA?

I've got D too

(1)
12C(n-m+1)=13
(n-m+1)=13
n-m=12
12C12 = 1
Suff

(2)
n= m + 12
this is actually the same as (1)
n-m=12
12C12 = 1
Suff

D