Aredox reaction has occurred

a. when a substance reacts with elemental oxygen.
b. when an alkali metal reacts with an alkaline earth metal.
c. when one substance transfers protons to another.
d. when one substance forms a solid.
e. when all of the above occur.

Answer : The correct option is, (a) when a substance reacts with elemental oxygen.

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.

From the given options we conclude that a redox reaction has occurred  when a substance reacts with elemental oxygen. Because elemental oxygen will oxidize the metal and itself get reduced. Thus, it is a redox reaction.

For example :

This reaction is a redox reaction in which magnesium shows oxidation due to change in oxidation state from (0) to (+2) and oxygen shows reduction due to change in oxidation state from (0) to (-2).

Hence, the correct option is, (a) when a substance reacts with elemental oxygen.