Which of the following is not an example of oxidizing agent?

 An oxidizing agent is an element that gains electrons. Since the oxidizing agent means to gain electrons; it is said to have been reduced. The element which undergoes reduction (gets reduced) is called an oxidizing agent.

Ozone, Hydrogen peroxide, and Nitric acid are oxidizing agents.

Carbon undergoes oxidation (gets oxidized) so, it is called a reducing agent.

Hence, Carbon is not an example of an oxidizing agent.