The correct option is D both (a) and (b)
774−474 is divisible by 3
since an−bn is divisible by (a - b) and
Again 774−474=(72)37−(42)37=4937−1637
Now, since an−bn is divisible by (a - b) if n is odd, then 774−474 (i.e., 4937−1637) is divisible by 11.
Hence d.