Reducing power runs B > C > A — option 2.

The principle. A reducing agent is a substance that gives up electrons and is itself oxidised. The standard electrode potential quoted for a metal is its reduction potential; the more negative it is, the less willing the ion is to accept electrons, and therefore the more readily the metal loses them. So :

the more negative the standard reduction potential, the stronger the reducing agent.

Arranging in decreasing order of reducing power gives B > C > A.

A check against a familiar case. Lithium has the most negative standard reduction potential of all, about −3·04 V, and is indeed among the strongest reducing agents known; fluorine has the most positive, about +2·87 V, and is the strongest oxidising agent. Metal B, at −3 V, is therefore behaving like an alkali metal, and metal A, at +0·5 V, like a noble metal such as copper or silver.

The link with the reactivity series. The electrochemical series is simply the reactivity series arranged by electrode potential. A metal higher in it — more negative — will displace one lower down from its salt solution, will react more vigorously with acids, and corrodes more readily. So metal B would displace both C and A from their solutions, and C would displace A.

The common error is to read the ranking straight from the numbers as printed, taking the largest value as the strongest. That gives A > C > B — option 1, placed there for precisely that mistake. Remember that oxidising power follows the positive direction and reducing power the negative.

Hence, the answer is B > C > A.