Concept used

For a metal to act as a reducing agent, it must itself get oxidized easily: $M(s) \to M^{n+} + ne^-$

The stronger the reducing agent, the greater its tendency to undergo oxidation.

We are given standard reduction potentials: $M^{n+} + ne^- \to M(s)$

If the reduction potential is more negative, then the reverse oxidation process is more favorable. Hence, more negative $E^\circ_{\text{red}}$ means stronger reducing power of the metal.

Given standard reduction potentials

$Zn^{2+} + 2e^- \to Zn, \quad E^\circ = -0.76\,V$ $Ca^{2+} + 2e^- \to Ca, \quad E^\circ = -2.87\,V$ $Mg^{2+} + 2e^- \to Mg, \quad E^\circ = -2.36\,V$ $Ni^{2+} + 2e^- \to Ni, \quad E^\circ = -0.25\,V$

Compare reducing powers

Since more negative reduction potential means stronger reducing agent:

• $Ni$ has $E^\circ = -0.25\,V$ → weakest reducing agent
• $Zn$ has $E^\circ = -0.76\,V$
• $Mg$ has $E^\circ = -2.36\,V$
• $Ca$ has $E^\circ = -2.87\,V$ → strongest reducing agent

Therefore, increasing reducing power: $Ni < Zn < Mg < Ca$

Match with options

This corresponds to Option B.

Verification with stored answer

Stored correct answer: B

Our derived answer: B

So, the answer agrees with the stored answer.