Explanation:

Mechanism of Reaction:
The order of a reaction is determined by the mechanism of the reaction as well as the relative concentration of the reactants. The mechanism of a reaction refers to the series of steps that occur during the transformation of reactants into products. The rate-determining step of the mechanism typically determines the overall order of the reaction. The rate-determining step is the slowest step in the mechanism and involves the breaking and forming of bonds.

Relative Concentration of Reactants:
The relative concentration of the reactants also plays a crucial role in determining the order of a reaction. The order of a reaction with respect to a particular reactant is the power to which its concentration is raised in the rate equation. For example, if the rate equation is rate = k[A]^2[B], the reaction is second order with respect to reactant A and first order with respect to reactant B.

Temperature:
While temperature can affect the rate of a reaction, it does not directly determine the order of the reaction. However, temperature can influence the rate constant (k) in the rate equation and thereby impact the overall rate of the reaction.

Molecularity:
The molecularity of a reaction refers to the number of molecules that participate in the rate-determining step of the reaction. Molecularity is only applicable to elementary reactions, which occur in a single step. The overall order of a reaction can be different from its molecularity, as the rate-determining step may involve multiple molecules.
Therefore, the correct answer is option B, as the mechanism of the reaction and the relative concentration of the reactants are key factors in determining the order of a reaction.