Assertion A: When hydrogen and nitrogen combines, ammonia is formed.
Reason(R):It is an exothermic reaction.

Options

A. Both A and R are true, and R is the correct explanation of A.
B. Both A and R are true, but R is not the correct explanation of A.
C. A is true but R is false.
D. A is false but R is true.

Explanation

To solve the question regarding the assertion and reason, we will analyze both statements step by step.

### Step 1: Analyze the Assertion (A)
The assertion states: "When hydrogen and nitrogen combine, ammonia is formed."

- **Chemical Reaction**: The chemical equation for the formation of ammonia (NH₃) from hydrogen (H₂) and nitrogen (N₂) is:
\[ N_2(g) + 3H_2(g) \rightarrow 2NH_3(g) \]
- **Conclusion**: This assertion is correct as the reaction does indeed produce ammonia when hydrogen and nitrogen react together.

### Step 2: Analyze the Reason (R)
The reason states: "It is an exothermic reaction."

- **Exothermic Reaction**: An exothermic reaction is one that releases energy, usually in the form of heat. The formation of ammonia from hydrogen and nitrogen is an exothermic reaction, and it releases approximately 92 kJ of energy.
- **Conclusion**: This reason is also correct, as the reaction does release energy.

### Step 3: Determine the Relationship Between A and R
Now we need to determine if the reason (R) correctly explains the assertion (A).

- **Correctness of Explanation**: While both statements are true, the reason provided does not explain why hydrogen and nitrogen combine to form ammonia. The formation of ammonia requires specific conditions, such as the presence of a catalyst (like iron) and high temperature and pressure. The reason given (that it is exothermic) does not address these necessary conditions for the reaction to occur.

### Final Conclusion
- **Assertion (A)**: True
- **Reason (R)**: True
- **Explanation**: R is not the correct explanation for A.

Thus, the correct answer is that both A and R are true, but R is not the correct explanation of A.