Assertion : Electromeric effect is brought into play only at the requirement of the reagent.
Reason : It is a temporary effect in which bond pair is shifted to one of the constituent atoms.

Options

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

Explanation

To solve the question, we need to analyze the assertion and reason provided:

**Assertion:** Electromeric effect is brought into play only at the requirement of the reagent.
**Reason:** It is a temporary effect in which bond pair is shifted to one of the constituent atoms.

### Step-by-Step Solution:

1. **Understanding the Electromeric Effect:**
- The electromeric effect is a temporary effect observed in organic chemistry where the electron density in a molecule shifts due to the presence of an attacking reagent (electrophile or nucleophile).
- It is important to note that this effect only occurs when a reagent is present that can interact with the molecule.

**Hint:** Recall that the electromeric effect is a response to external reagents.

2. **Analyzing the Assertion:**
- The assertion states that the electromeric effect is activated only when there is a requirement for a reagent.
- This is true because the effect is dependent on the presence of an electrophile or nucleophile that can interact with the molecule.

**Hint:** Consider how the presence of reagents influences molecular interactions.

3. **Analyzing the Reason:**
- The reason explains that the electromeric effect is temporary and involves the shifting of a bond pair of electrons to one of the atoms in the bond.
- This is also true, as the effect is indeed temporary and occurs during the interaction with the reagent.

**Hint:** Think about how electron pairs behave during chemical reactions.

4. **Conclusion:**
- Both the assertion and reason are true.
- The reason correctly explains the assertion, as the electromeric effect is indeed a temporary phenomenon that occurs when reagents are present.

**Hint:** Reflect on how assertions and reasons relate to each other in scientific explanations.

5. **Final Answer:**
- Since both the assertion and reason are true, and the reason provides a correct explanation for the assertion, the correct choice is that both are true and the reason is a correct explanation of the assertion.

### Final Answer:
Both the assertion and reason are true, and the reason is a correct explanation of the assertion.