Assertion. In a combustion reaction in the air, oxygen is the limiting reactant.
Reason. Oxygen is present in limited amount (only `21%`) in the air.

Options

A. If both assertion and reason are true, and reason is the true explanation of the assertion.
B. If both assertion and reason are true, but reason is not the true explanation of the assetion.
C. If assertion is true, but reason if false
D. If both assertion and reason are false.

Explanation

To analyze the assertion and reason provided in the question, we will break it down step by step.

### Step 1: Understand the Assertion
The assertion states that "In a combustion reaction in the air, oxygen is the limiting reactant."

**Explanation**: A limiting reactant is the substance that is completely consumed in a chemical reaction, thus determining the maximum amount of product that can be formed. In combustion reactions, oxygen is typically required for the reaction to occur.

### Step 2: Understand the Reason
The reason given is that "Oxygen is present in limited amount (only 21%) in the air."

**Explanation**: While it is true that oxygen makes up about 21% of the air, this does not mean it is in limited supply for combustion reactions. In many cases, the amount of oxygen available in the air is more than sufficient for the combustion of common substances.

### Step 3: Analyze the Relationship Between Assertion and Reason
Now, we need to determine if the assertion is true or false based on the reason provided.

- In typical combustion reactions, oxygen is not the limiting reactant because it is present in excess in the atmosphere. Even though it constitutes only 21% of air, there is still a large volume of oxygen available for combustion.
- Therefore, the assertion is **false**.

### Step 4: Conclusion
Both the assertion and the reason are false. Oxygen is not a limiting reactant in combustion reactions occurring in air since it is present in excess.

### Final Answer
Both the assertion and reason are false.

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