Assertion (A): Hydrolysis of an ester follows first order kinetics.
Reason (R): Concentration of water remains nearly constant during the course of the reaction.

Options

A. Both A and R are true and R is the correct explanation for A
B. Both A and R are true but R is NOT the correct explanation of A
C. A is true but R is false
D. A is false and R is true

Explanation

To analyze the assertion and reason provided in the question, we will break down the concepts involved in the hydrolysis of an ester and the kinetics of the reaction.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that the hydrolysis of an ester follows first-order kinetics.
- Hydrolysis of an ester can be represented as:
\[ \text{RCOOR'} + \text{H}_2\text{O} \rightarrow \text{RCOOH} + \text{R'OH} \]
- In this reaction, the ester reacts with water to form an acid and an alcohol.

2. **Understanding the Reason (R)**:
- The reason states that the concentration of water remains nearly constant during the course of the reaction.
- In many hydrolysis reactions, especially in dilute solutions, the concentration of water is much larger than that of the ester. Therefore, it can be treated as a constant.

3. **Kinetics of the Reaction**:
- For a reaction to be first-order, the rate of reaction should depend linearly on the concentration of one reactant.
- In the case of ester hydrolysis, if we consider water to be in excess, the rate law can be simplified to:
\[ \text{Rate} = k'[\text{Ester}] \]
- Here, \( k' \) is the pseudo-first-order rate constant, which incorporates the constant concentration of water.

4. **Conclusion**:
- Since the hydrolysis of an ester does indeed follow first-order kinetics under the condition that water is in large excess, and the reason provided correctly explains this behavior, both the assertion and the reason are true, and the reason is a correct explanation of the assertion.

### Final Answer:
Both Assertion (A) and Reason (R) are true, and R is the correct explanation for A.