Final answer:
In the reactivity triangle, the fuel source for the reaction is the reducing agent, which donates electrons. The correct option is B.
Step-by-step explanation:
In the reactivity triangle, the fuel source for the reaction is known as the reducing agent. This component donates electrons in a redox reaction, effectively fueling the chemical process. In regards to activation energy, it provides the necessary "push" to start a chemical reaction.
The source of activation energy is usually heat energy from the surroundings. By increasing the motion of molecules, heat energy raises the frequency and force of molecular collisions, contributing to the transition state necessary for reactions to proceed.
When looking at the list provided in the question, the oxidizing agents would typically be molecules that accept electrons during the redox reaction. Examples of such agents include compounds like FAD and NAD+, which gain electrons and are reduced in the process. Hence, B is the correct option.