In order to react, a molecule, at the time of collision, must possess a certain amount of energy known as:

threshold energy

The correct answer is option 3. threshold energy.

Threshold Energy is the minimum Kinetic Energy the molecules must have to bring about effective collisions between two reactant molecules which are considered spheres, thus resulting in a chemical reaction between the two reactants. Therefore, we can write:

Threshold Energy = Average of the initial Kinetic Energy of reactants + Activation Energy (E_a)

If the reactants at the initial state (start of the reaction) have sufficient energy for the collisions, then the threshold energy is equal to Activation Energy (E_a).

Molecules with a Kinetic Energy equal to or higher than the Threshold Energy will react.