Why is `BiH_(3)` the strongest reducing agent amongst all the hydrides of group `15` elements ?

Explanation

As we move down the group, the size of the element increases and, therefore, the length of the E-H bond increases and its strength decreases. In other words, as we move down the group, the E-H bond can break more easily to evolve `H_(2)` gas which acts as the reducing agent. Thus, Bi-H bond is the weakest amongst the hydrides of elements of group 15 and hence `BiH_(3)` is the strongest reducing agent.