Which of the following represents an exothermic reaction :-

Options

A. `N_(2)(g)+O_(2)(g)rarr 2NO(g), Delta H=180.5 KJ`
B. `H_(2)O(g)+C(s)rarr CO(g)+H_(2)(g), Delta E=131.2KJ`
C. `2HgO(s)+180.4 KJ rarr 2Hg(l)+O_(2)(g)`
D. `2Zn(s)+O_(2)(g)rarr 2ZnO(s), Delta E = -693.8 KJ`

Answer

To determine which of the following represents an exothermic reaction, we need to understand the definitions of exothermic and endothermic reactions.

### Step-by-Step Solution:

1. **Understand Exothermic Reactions**:
- An exothermic reaction is one that releases energy, usually in the form of heat. This can be represented by a negative change in enthalpy (ΔH < 0).

2. **Understand Endothermic Reactions**:
- An endothermic reaction absorbs energy from the surroundings, leading to a positive change in enthalpy (ΔH > 0).

3. **Analyze the Given Reactions**:
- For each reaction provided, we need to determine whether the change in enthalpy (ΔH) is positive or negative.
- If ΔH is negative, the reaction is exothermic; if ΔH is positive, the reaction is endothermic.

4. **Evaluate Each Option**:
- **Option A**: If ΔH is positive, this is an endothermic reaction.
- **Option B**: If ΔH is positive, this is also an endothermic reaction.
- **Option C**: If ΔH is positive, this is again an endothermic reaction.
- **Option D**: If ΔH is negative, this indicates that energy is released, making this an exothermic reaction.

5. **Conclusion**:
- The only option that represents an exothermic reaction is **Option D**, where energy is released during the formation of products.

### Final Answer:
The correct answer is **Option D**.