Final answer:
Reducing agents are mixed with oxidizing agents to cause an exothermic reaction. Oxidizing agents need a substance to donate electrons, allowing combustion reactions such as the oxidation of methane into carbon dioxide and water.
Step-by-step explanation:
The substance mixed with oxidizing agents that are not capable of exothermic decomposition to cause an exothermic reaction to take place is C) Reducing agents. Oxidizing agents, like potassium dichromate, require a chemical counterpart that can donate electrons to facilitate a reaction. Reducing agents serve this function by being oxidized themselves while reducing the oxidizing agent.
A common example of this type of reaction is the combustion of methane (CH4), where methane acts as a reducing agent and is oxidized to carbon dioxide (CO2) and water (H2O), with oxygen (O2) as the oxidizing agent. This type of reaction is highly exothermic and releases a significant amount of energy.