Standard electrode potential of three metals $X,Y$ and $Z$ are $-1.2\text{V},+0.5\text{V}$ and $-3.0\text{V}$ respectively. The reducing power of these metals will be:

Reducing power is inversely proportional to the standard reduction potential. The lower (more negative) the reduction potential, the stronger the reducing power (as the metal is more easily oxidized).Order of $E^{\circ }$: $Z(-3.0)<X(-1.2)<Y(+0.5)$ Order of Reducing Power: $Z>X>Y$.