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

Step-by-step solution

In group 15 ] P N → Non-metal ] Sb As → Metalloid Bi ] → Metal Hydrides of group 15 elements are NH 3 PH 3 AsH 3 SbH 3 BiH 3 In NH 3 , hydrogen atom gets partial positive charge due to less electronegativity. But in BiH 3 , hydrogen atom gets partial negative charge because hydrogen is more electronegative than bismuth. i.e. BiH 3 is a strong reducing agent than others because we know that H - is a strong reducing agent.