A){ 3 , 7, 8}
B){1, 3, 5, 7, 8}
C){ 3}

Answer:
since B only has 1, 5 3
and A has 1, 5, 4, 6, 2,
and they both share 1, 5
B - A will leave B with only 3
it's letter C
I think
correct me if I'm wrong don't put this answer
the question is a little vague