Exothermic reactions release energy. Where is the energy used for these reactions to occur stored? .

In the chemical bonds.

Explanation:

There are different types of chemical bonds, some store a TON of energy while others do not store as much. You can think of it like this: for bonds with low energy, it takes a LOT of force to pull them apart, meaning they are strong bonds. However, this way, you have to put energy into them to break the bonds and form a chemical reaction. This is an endothermic reaction, a reaction that absorbs heat from its surroundings.

Instead, bonds in exothermic reactions are BEGGING to form a more stable bond, and hence store a lot of potential energy. Due to the difficulty of maintaining these bonds, there is a lot of energy put into them. If you break them, however, (exothermic reaction) the energy is released, and hence the exothermic reaction happens.

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it is b that is the answer.