“Reactants” must "collide" for a "chemical reaction" to proceed.
Option: B
Explanation:
Collision theory explains why collision of reactant is primary stage for any "chemical reaction" to take place. According to it “rate of reaction is proportional to the rate of reactants collisions”. Orientation of reacting species during collision is important to result into product, where atoms are bonded together. Adequate amount of energy is compulsory to allow mutual penetration of reacting species valence shells for rearrangement of electrons and allow formation of new bonds. While activated complex (which is unstable species) formation is transition state of chemical reaction having very short life and mostly undetectable.