How can you tell for sure that a redox reaction has happened?

Determine the value of the oxidation numbers of atoms or ions on the reactant side and the product side.

Explanation:

A redox reaction involves some ions, and atoms losing electrons and other gaining them in the same reaction. When oxidation occurs, there is loss of electrons by an element thus its oxidation number increases; becomes more positive. When reduction occurs an element gains electrons and the oxidation  number becomes more negative; reduces.

If an element (s) behaves in this manner in the whole reaction, then be sure to deduce that a redox reaction has occurred.

the answer is definitely c.