Why `BiH_3` is strongest reducing agent amongst group `15` hydrides ?

Explanation

As we move down the group from N to Bi, the size of the element increases. As a result, bond length increases and bond strength decrease from N to Bi. In other words, Bi-H bond is the weakest and hence decomposes, on heating, to liberate `H_(2)` most readily. Thus, `BiH_(3)` is the strongest reducing agent in group 15 element hydrides.