A={2,4,7,8} B={3,6,7,8}
C shares exactly 2 elements with A as well as with B, and they could be the same too.
1. 6 is not in set C
C={7,8,10,11} or C={2,4,3,8}
In first case: 2 elements shared
In second case: 1 element shared
Not sufficient
2. 3 is in set C
C={3,6,2,4} or C={3,10,8,4}
First case: None shared
Second case: 1 element shared
Not sufficient
Together 1 and 2 combined
A={2,4,7,8} B={3,6,7,8}
6 is NOT in set C and 3 IS IN Set C
Set C contains 3 but cannot have 6, so it has to have either 7 or 8 so as to meet the condition that 2 elements have to be common between B and C
So considering it now:
If 2nd element in C is 7/8: C = {3,7,n1,n2}, now for it to also match with 2 elements common with Set A condition, it needs to have either 2 or 4 in it because it already has 8 in common with Set A and it cannot have 7 because then Set C will have 3 in common with Set B which cannot happen.
So C = {3,7,2,100}/{3,7,4,100}/{3,8,2,100}/{3,8,4,100} and in all these cases there is only 1 element common between all 3 sets
SUFFICIENT
Answer - C