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

Explanation

Doen the group `(darr)` i.e, from N to Bi, the atomic size increases, hence the bond length `A-H` increases consequently `A-H` bond strength decreases and tendency to behave as reducing agent increases i.e., `BiH_3` is the strongest reducing agent amongst all the hydrides of group 15 elements.