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, the elements are N, P, As, Sb, and Bi. Among these, the metallic character increases as we go down the group. Bismuth (Bi) is the most metallic element in Group 15.

The hydrides of Group 15 elements are $$NH_3$$, $$PH_3$$, $$AsH_3$$, $$SbH_3$$, and $$BiH_3$$. The reducing power of these hydrides increases down the group because the bond strength between the central atom and hydrogen decreases as the size of the central atom increases. Since $$BiH_3$$ (bismuthine) has the weakest Bi-H bond, it is the most easily decomposed and hence the strongest reducing agent among all Group 15 hydrides.

Therefore, the element that is a metal and forms the hydride with the strongest reducing power is Bi (Bismuth). The correct answer is Option (4): Bi.