A: The process of demodulation is carried out to retrieve the message signal.
R: The range of the line-of -sight propagatin is limited mainly due to earth's curvature.

Answer

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

### Step 1: Analyze the Assertion
- **Assertion (A)**: "The process of demodulation is carried out to retrieve the message signal."
- **Explanation**: Demodulation is indeed the process used in communication systems to extract the original message signal from the modulated carrier wave. This process is essential for the receiver to understand the transmitted information.

### Step 2: Analyze the Reason
- **Reason (R)**: "The range of line-of-sight propagation is limited mainly due to earth's curvature."
- **Explanation**: In line-of-sight communication, the transmitter and receiver must be able to "see" each other directly. The curvature of the Earth limits this direct line of sight, as the signal can be obstructed by the Earth's surface. Therefore, this statement is also true.

### Step 3: Determine the Relationship Between Assertion and Reason
- Both the assertion and reason are true statements. However, we need to assess whether the reason explains the assertion.
- **Explanation**: While both statements are true, the reason (R) does not provide an explanation for the assertion (A). The assertion discusses the process of demodulation, while the reason discusses the limitations of line-of-sight communication.

### Conclusion
- Since both statements are true but the reason does not explain the assertion, the correct answer is that the assertion is true, the reason is true, but the reason does not explain the assertion.

### Final Answer
- The correct option is: **A is true, R is true, but R does not explain A.**

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