Assertion : The formula of sodium carbonate is `Na_2CO_3`.
Reason : Na is divalent, `CO_3` is monovalent.

Options

A. If both assertion and reason are true and reason is the correct explanation
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 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 formula of sodium carbonate is `Na2CO3`.
- To verify this, we need to know the ions that make up sodium carbonate.

2. **Identifying the Ions**:
- Sodium (Na) forms a sodium ion with a charge of +1, represented as `Na+`.
- The carbonate ion (CO3) has a charge of -2, represented as `CO3^2-`.

3. **Balancing the Charges**:
- To form a neutral compound, the total positive charge must balance the total negative charge.
- Since the carbonate ion has a charge of -2, we need two sodium ions (each with a +1 charge) to balance it out:
- Total positive charge from 2 `Na+` = +2
- Total negative charge from 1 `CO3^2-` = -2
- Therefore, the formula `Na2CO3` is correct.

4. **Understanding the Reason**:
- The reason states that sodium (Na) is divalent and CO3 is monovalent.
- However, we established that sodium is actually monovalent (with a +1 charge) and the carbonate ion (CO3) is divalent (with a -2 charge).

5. **Conclusion**:
- The assertion is true because the formula for sodium carbonate is indeed `Na2CO3`.
- The reason is false because sodium is not divalent; it is monovalent.
- Therefore, the correct conclusion is that the assertion is true, but the reason is false.

### Final Answer:
- The assertion is true, and the reason is false.