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

Explanation

In group 15 elements, Bi has the largest atomic size and the least tendency to form covalent bond with smell hydrogen atom in `BiH_(3)`.
(2) Bond dissociation enthalpy of `Bi-H` bond is minimum among all hydrides of group 15 elements.
(3) `BiH_(3)` is the least stable hybrides in group 15.
(4) Therefore `BiH_(3)` can readily supply H-atom for the reduction. Hence `BiH_(3)` is the strongest reducing agent.