According to the collision theory, when can a chemical reaction occur?

when enough activation energy is added to correct the orientation of the particle collisions
when reactants collide with enough energy to intersect their valence shells and form new bonds
when reactants collide with enough mass to form new bonds and break apart the reactants
when the proper catalyst is added to break the chemical bonds in the reactants.

when reactants collide with enough energy to intersect their valence shells and form new bonds

The correct answer is option A, When enough activation energy is added to correct the orientation of the particle collisions

Explanation:

According to collision theory the two most important factors behind the occurrence of any chemical reaction are –  

a)Sufficient energy  to collide – The activation energy of the colliding particles should be able to avoid bouncing back of colliding particle when they approach towards each other. Also this activation energy is more or less referred to as kinetic energy which increases with the increasing temperature.  

b)Molecular Orientation – Even if the colliding particles have enough energy, they sometime do not react with each other as their orientation of approach is not correct / perfect.