Assertion : The rate of reaction is the rate of change of concentration of a reactant or a product.
Reason : Rate of reaction remians constant during the complete reaction.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To solve the given question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "The rate of reaction is the rate of change of concentration of a reactant or a product."
- This is a fundamental concept in chemical kinetics. The rate of a reaction is defined as the change in concentration of a reactant or product over a specific time interval.
- Mathematically, it can be expressed as:
\[ \text{Rate} = \frac{\Delta [\text{Reactant/Product}]}{\Delta t} \]
- Therefore, the assertion is **true**.

2. **Understanding the Reason**:
- The reason states that "Rate of reaction remains constant during the complete reaction."
- This statement is incorrect. The rate of a reaction typically changes over time as the concentrations of reactants decrease and products increase during the course of the reaction.
- As the reaction progresses, the rate can vary due to changes in concentration, which affects how quickly reactants are converted to products.
- Therefore, the reason is **false**.

3. **Conclusion**:
- Since the assertion is true and the reason is false, the correct answer is that the assertion is true but the reason is false.

### Final Answer:
- The correct option is **C**: Assertion is true but reason is false.