Question

If k is a number so that the sum of k and any number in the set above is also in the set, what is the value of k?

Answer 1

k=0 because any other answer does not work, they all are not in the parentheses above. 0 plus every number is the same number, which is still above.

Answer 2

Just by trying out the options,

K = 0, 
0 plus any of the number (6,2,1,3) still results to 6,2,1,3 which is in the set.

option (D) is the answer.