Assertion : The enthalpy change for the reaction `CaO_((s))+CO_(2(g)) rarr CaCO_(3(s))` is called enthalpy of formation of calcium carbonate.
Reason : The reaction involves formation of 1 mole of `CaCO_(3)` from its constituent elements.

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 analyze the assertion and reason provided in the question, we will break down the concepts of enthalpy of formation and the specifics of the given reaction:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the enthalpy change for the reaction \( \text{CaO}_{(s)} + \text{CO}_{2(g)} \rightarrow \text{CaCO}_{3(s)} \) is called the enthalpy of formation of calcium carbonate.
- **Evaluation**: The enthalpy of formation is defined as the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states.

2. **Identifying the Reaction Components**:
- In the reaction, we have:
- Calcium oxide (\( \text{CaO} \)) which is a compound.
- Carbon dioxide (\( \text{CO}_2 \)) which is also a compound.
- Calcium carbonate (\( \text{CaCO}_3 \)) is the product formed.
- **Observation**: The reactants are not the elements in their standard states; they are compounds.

3. **Conclusion on the Assertion**:
- Since the reaction does not involve the formation of calcium carbonate from its constituent elements (calcium, oxygen, and carbon), the assertion is incorrect.

4. **Understanding the Reason**:
- The reason states that the reaction involves the formation of one mole of \( \text{CaCO}_3 \) from its constituent elements.
- **Evaluation**: As previously established, the reactants are compounds, not elements. Therefore, this statement is also incorrect.

5. **Final Conclusion**:
- Both the assertion and the reason are incorrect. Thus, the correct answer is that both statements are false.

### Final Answer:
Both the assertion and the reason are incorrect.