To understand why the correct answer is option C (bimolecular reactions), let's first define what effective collisions are in the context of chemical reactions.

Effective collisions are those collisions between molecules that result in a chemical reaction taking place. Not all collisions between molecules lead to a reaction, as certain conditions must be met for a reaction to occur. One of these conditions is that the colliding molecules must possess enough energy to overcome the activation energy barrier for the reaction.

Now let's look at the different options provided and analyze why option C is the correct answer.

a) First order reactions:
First order reactions refer to reactions where the rate of the reaction depends on the concentration of a single reactant. While the concentration of reactants can influence the rate of a reaction, it does not necessarily determine whether an effective collision occurs or not. Therefore, first order reactions alone do not guarantee effective collisions.

b) Second order reactions:
Second order reactions refer to reactions where the rate of the reaction depends on the concentration of two reactants. Similar to first order reactions, the concentration of reactants can influence the rate of a reaction, but it does not determine whether an effective collision occurs or not. Therefore, second order reactions alone do not guarantee effective collisions.

c) Bimolecular reactions:
Bimolecular reactions refer to reactions involving the collision between two molecules. In order for effective collisions to occur, the colliding molecules must possess enough energy to overcome the activation energy barrier. Bimolecular reactions have a higher likelihood of leading to effective collisions because the collision between two molecules increases the chances of having enough energy to overcome the activation energy barrier.

d) Zeroth order reactions:
Zeroth order reactions refer to reactions where the rate of the reaction is independent of the concentration of the reactants. In these reactions, the concentration of the reactants does not influence the rate of the reaction, and therefore, it does not determine whether effective collisions occur or not.

In conclusion, the correct answer is option C (bimolecular reactions) because the collision between two molecules increases the chances of having enough energy to overcome the activation energy barrier, leading to effective collisions.