Why is `BiH_3` the strongest reducing agent amongst all the hydrides of group 15 elements?

Explanation

As we move down a group, the atomic size increases and the stability of the hydrides of group 15 elements decreases. Since the stability of hydrides decreases on moving from `NH_(3)` to `BiH_(3)`, the reducing character of the hydrides increases on moving from `NH_(3)` to `BiH_(3)`.