Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Heavy water is used for the study of reaction mechanism.
Reason (R): The rate of reaction for the cleavage of O-H bond is slower than that of O-D bond.
Choose the most appropriate answer from the options given below:

Heavy water is $$\text{D}_2\text{O}$$ in which the ordinary hydrogen $$({^1\text{H}})$$ is replaced by its heavier isotope deuterium $$({^2\text{H}} = \text{D})$$.

Case 1: Checking Assertion (A).
Because deuterium behaves chemically like protium yet can be detected separately, replacing $$\text{H}$$ with $$\text{D}$$ in a reactant makes it possible to trace which bonds break and which intermediates form. Such isotopic labelling studies help chemists determine detailed reaction mechanisms. Hence, Assertion (A) is true.

Case 2: Checking Reason (R).
Bond strength increases with atomic mass since the zero-point vibrational energy decreases. Therefore the $$\text{O-D}$$ bond is stronger than the $$\text{O-H}$$ bond. Because a stronger bond is harder to break, reactions that involve cleavage of $$\text{O-D}$$ proceed more slowly than those that cleave $$\text{O-H}$$. Mathematically this is the primary kinetic isotope effect: $$k_{\text{O-H}} \gt k_{\text{O-D}}$$. The statement given in (R) says “the rate of reaction for the cleavage of O-H bond is slower than that of O-D bond,” which is the opposite of the correct relationship. Thus, Reason (R) is false.

Case 3: Relation between (A) and (R).
Since (A) is true and (R) is false, (R) cannot be the correct explanation of (A).

Therefore the correct option is Option B: (A) is true but (R) is false.