The answer would be: A. a=0
The result of the equation would be undefined if the denominator is zero. In this question, the denominator is (b)(b-1)(b+1). You can find 3 results of variable b that will result in zero value of denominator.
(b)(b-1)(b+1)=0
1. b=0
2. b-1=0
b=-1
3. b+1=0
b=-1