Which equation describes a reduction?
mg(s)—mg2+ (aq) + 2e -
201 + 2e -> 201
na(s) — nat(aq) + e
al(s)— a3+ (aq) + 3e -

1,3,4 equations represent oxidation as charge increases

Your 2 nd option represents reduction

Answer : The reduction will be

Explanation :

Redox reaction or Oxidation-reduction reaction : It is defined as the reaction in which the oxidation and reduction reaction takes place simultaneously.

Oxidation reaction : It is defined as the reaction in which a substance looses its electrons. In this, oxidation state of an element increases. Or we can say that in oxidation, the loss of electrons takes place.

Reduction reaction : It is defined as the reaction in which a substance gains electrons. In this, oxidation state of an element decreases. Or we can say that in reduction, the gain of electrons takes place.

(a) The given half reactions is:

This reaction is an oxidation reaction because in this reaction, the loss of electrons takes place.

(b) The given half reactions is:

This reaction is an reduction reaction because in this reaction, the gain of electrons takes place.

(c) The given half reactions is:

This reaction is an oxidation reaction because in this reaction, the loss of electrons takes place.

(d) The given half reactions is:

This reaction is an oxidation reaction because in this reaction, the loss of electrons takes place.

Hence, the reduction will be