ANS:A - 4

If we look at the possibilities involved in subtracting one 1-bit number from another, we can quickly see that three of the four possible combinations are easy and straight-forward. The fourth one involves a bit more:

0 - 0 = 0
1 - 0 = 1
1 - 1 = 0
0 - 1 = 1, with a borrow bit.