Plz explain the question

Two n bit binary strings, S, and S, are chosen randomly with uniform probability. The probability that the Hamming distance between these strings (the number of bit positions where the two strings differ) is equal to d is C /2 C/2 (A) d/2 (B) (D) 1/2d (C) 1 sarandincla comlCall Toll Free: 1800-120-1021