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

In group $15$

${}_{\mathrm{P}}{}^{\mathrm{N}}]\to$ Non-metal

${}_{\mathrm{Sb}}{}^{\mathrm{As}}]\to$ Metalloid

$\mathrm{Bi}]\to$ Metal

Hydrides of group 15 elements are

${\mathrm{NH}}_3$

${\mathrm{PH}}_3$

${\mathrm{AsH}}_3$

${\mathrm{SbH}}_3$

${\mathrm{BiH}}_3$

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.