If the state machine described in figure should have a stable state, the restriction on the inputs is given by
Answer is B)
@Swapnil Naik stable state means after certain time(after all delays) it gets stable and doesn't change. For above diagram to make sure output does not change, at S1 we should apply a=1 and at S2 we should apply b=1. Any other input will make our output fluctuate between S1 and S2.