In a redox reaction, reduction is defined by the:

a. gain of electrons, resulting in an increased oxidation number.
b. loss of electrons, resulting in a decreased oxidation number.
c. gain of electrons, resulting in a decreased oxidation number.
d. loss of electrons, resulting in an increased oxidation number.

C. Gain in electron(s) resulting in a decrease of oxidation number.

Explanation:

Redox reactions are reactions involving transfer of of electron between two species (reduction specie) and (oxidation species) and change resulting in change in oxidation number.

Reduction in terms of redox reaction is the specie that accepts electron(s) and gets "reduced" since its oxidation state has been reduced.

For example

Cl + e- → Cl⁻

The above reaction is an example of reduction reaction taking place in a redox reaction. We can see that Chlorine oxidation state was changed from (0) to (-1) state.