The strongest reducing agent amont the following is:

The reducing character of the hydrides of Group $15$ elements increases from ${NH}_{3}$ to ${BiH}_{3}$ (Bismuthine) because the reducing character depends upon the stability of the hydride.The greater the unstability of hydride,the greater is the its reducing character. Since,the ${BiH}_{3}$ is the least stable (because the size of central atom is greatest & therefore its tendency to form stable covalent bond with small hydrogen atom decreases,as a result the bond strength decreases) in this series,hence the reducing character increases.