What must happen before a chemical reaction can begin?

The correct answer is that the activation energy must be reached.

Explanation:

In order to start a chemical reaction, energy is needed. A certain amount of energy is needed in order to get started. This energy is termed as activation energy. A reaction would not take place unless the molecules or atoms of reactants come together. This takes place only when the particles are in motion, and for motion, energy is required.  

The reactants have to overcome forces, which push them away from each other, and the bonds present in the reactants are needed to get dissociated, for both these purpose energy is required. Thus, before the beginning of a chemical reaction, the activation energy must be reached.