Concept: The question tests on the application of using equations to reach at a conclusion in a DS format
We are asked if a-1/b+1 = a/ b
Or we are asked if b (a-1) = a(b+1) [On cross multiplying]
Or if b a –b = a b + a
= > If a = -b ?
St (1) a= b-4
If b= 2 then a= 2-4 = -2 and hence a is EQUAL to –b (Yes)
However if b =4 a=0 and a is NOT EQUAL to –b (No )
Hence, two contradictory answers and thus St (1) is Insufficient
St (2) a = -b
It’s exactly what we are asking for in the question stem.
Thus sufficient. Option (B)