Assertion: Steam is a mixture.
Reason: In a compound, the composition of the elements must be fixed.

Options

A. Assertion and reason both are correct statements and reason is correct explanation for assertion.
B. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
C. Assertion is correct statements but reason is wrong statement.
D. Assertion is wrong statement but reason is correct statement.

Answer

### Step-by-Step Solution

1. **Understanding the Assertion**: The assertion states that "Steam is a mixture." We need to evaluate if this statement is true or false.

2. **Definition of Steam**: Steam is the gaseous phase of water (H2O). It is produced when water is heated and turns into vapor.

3. **Classification of Steam**: Since steam is composed of water molecules (H2O) in the vapor state, it is not a mixture but a pure substance. A mixture consists of two or more different substances that retain their individual properties, while steam is a single compound.

4. **Conclusion on Assertion**: Therefore, the assertion that "Steam is a mixture" is **false**.

5. **Understanding the Reason**: The reason states that "In a compound, the composition of the elements must be fixed."

6. **Definition of a Compound**: A compound is a substance formed when two or more elements are chemically bonded together in a fixed ratio. For example, water (H2O) is a compound made of hydrogen and oxygen in a 2:1 ratio.

7. **Fixed Composition**: The composition of elements in a compound is indeed fixed. For instance, in water (H2O), the ratio of hydrogen to oxygen is always 2:1, regardless of the amount of water.

8. **Conclusion on Reason**: Therefore, the reason that "In a compound, the composition of the elements must be fixed" is **true**.

9. **Final Evaluation**: Since the assertion is false and the reason is true, the correct conclusion is that the assertion and reason are not both true.

10. **Correct Option**: The correct option based on the evaluation is that the assertion is incorrect while the reason is correct.