Assertion (A). Combustion of all organic compounds is an exothermic reaction.
Reason (R). The enthalpies of all elements in their standard state are zero

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 assertion-reason question, we will analyze both the assertion and the reason separately and then determine their validity.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Combustion of all organic compounds is an exothermic reaction."
- **Definition of Combustion**: Combustion is a chemical reaction that occurs when a substance reacts with oxygen, typically producing heat and light. In organic compounds, this generally involves the reaction of hydrocarbons with oxygen to produce carbon dioxide and water.

2. **Analyzing the Exothermic Nature of Combustion**:
- In exothermic reactions, the total energy of the products is less than that of the reactants, leading to the release of energy (heat) to the surroundings.
- For combustion, the enthalpy change (ΔH) is negative, indicating that energy is released. Thus, the assertion is **true**.

3. **Understanding the Reason**:
- The reason states that "The enthalpies of all elements in their standard state are zero."
- **Standard State**: The standard enthalpy of formation for elements in their most stable form at 1 atm and 25°C is defined as zero. However, this does not directly explain why combustion is exothermic.

4. **Evaluating the Validity of the Reason**:
- While it is true that the enthalpy of elements in their standard state is zero, this fact alone does not provide a valid explanation for why combustion reactions are exothermic.
- The reason does not logically connect to the assertion about combustion being exothermic. Therefore, the reason is **false** in the context of explaining the assertion.

5. **Conclusion**:
- The assertion is true, but the reason is not a correct explanation for the assertion. Hence, the correct conclusion is that the assertion is true, but the reason is false.

### Final Answer:
- **Assertion (A)**: True
- **Reason (R)**: False
- Therefore, the correct option is that the assertion is true but the reason is not a correct explanation.