In ripple-carry adder, the carry "ripples" from one bit to the next. The longest path delay through n-bit ripple carry adder is 2n gate delays.
For this condition (longest latency) to arise, we must ensure that a carry that is generated at the least significant bit is propagated throughout till the last bit. This means we must have Cin = 1 for every stage of the ripple-carry adder.
Given A = +1 (in 2's complement form), we just need to find what should be B in 2's complement form to satisfy the given condition.
A= 0000 0001 (given)
B= 1111 1111 ( -1 in decimal)
On adding A+B we get Cin = 1 at every stage with the last Cout = 1. $\therefore$ Ans is -1.