This section contains
Assertion
(A) Carbon and oxygen combined together only in one fixed ratio.
Reason ( R) In a chemical compound, the elements are combined in a fixed ratio.

Options

A. Both Assertion and Reason are correct and Reason is the correct explanation of the Assertion
B. Both Assertion and reason are correct but Reason is not the correct explanation of Assertion
C. Assertion is correct but Reason is incorrect.
D. Both Assertion and Reason are incorrect

Answer

To solve the assertion and reason question, we need to analyze both the assertion (A) and the reason (R) provided.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that "Carbon and oxygen combined together only in one fixed ratio."
- This is incorrect. Carbon and oxygen can combine in different ratios to form different compounds. For example, carbon monoxide (CO) has a 1:1 ratio of carbon to oxygen, while carbon dioxide (CO₂) has a 1:2 ratio.

2. **Understanding the Reason (R)**:
- The reason states that "In a chemical compound, the elements are combined in a fixed ratio."
- This statement is generally true. In any chemical compound, the elements do combine in a fixed ratio by mass or by the number of atoms. However, this does not mean that all elements combine in only one fixed ratio.

3. **Evaluating the Relationship Between A and R**:
- The assertion is false because carbon and oxygen can combine in multiple ratios.
- The reason is true because it applies to all chemical compounds, but it does not support the assertion since the assertion is incorrect.

4. **Conclusion**:
- Since the assertion is false and the reason is true, the correct answer to the question is that the assertion is incorrect and the reason is correct.

### Final Answer:
- Assertion (A) is false.
- Reason (R) is true.
- Therefore, the correct option is that the assertion is incorrect while the reason is correct.