An oxidation-reduction reaction is also known as a redox reaction. what is one way you could tell that a redox reaction has occurred?
a. when one substance transfers electrons to another.
b. when a substance reacts with oxygen.
c. when a metal reacts with a nonmetal.
d. all of these would indicate a redox reaction.

a. When one substance transfers electrons to another.

Explanation:

The redox reaction occurs when one substance oxides and other reduces. To oxide, the compound must lose electrons, and to reduce, it must gain electrons. So, when it's observed that electrons are being transferred, we can tell that a redox reaction is happening.

The oxygen is a good oxidation agent and intends to reduce, but the combustion reaction, for example, involves oxygen and it's not a redox reaction. The reaction between a metal and a nonmetal not necessarily will be a redox reaction.