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

$\left.\begin{array}{l}\mathrm{N} \\ \mathrm{P}\end{array}\right] \rightarrow$ Non metal

$\left.\begin{array}{l}\text { As } \\ \text { Sb }\end{array}\right] \rightarrow$ Metalloid

$\mathrm{Bi}] \rightarrow$ 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.