A group 15 element, which is a metal and forms a hydride with strongest reducing power among group 15 hydrides.

In $\mathrm{NH}_{3}$, hydrogen atom gets partial positive charge due to less electronegativity. But in $\mathrm{BiH}_{3}$, hydrogen atom gets partial negative charge because hydrogen is more electronegative than bismuth.

i.e. $\mathrm{BiH}_{3}$ is a strong reducing agent than others because we know that $\mathrm{H}^{-}$ is a strong reducing agent.