Assertion (A) : LASER is used to measure distant object as moon.
Reason (R) : They are highly coherent source of light.

Options

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

Explanation

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided.

**Assertion (A):** LASER is used to measure distant objects such as the moon.

**Reason (R):** They are a highly coherent source of light.

### Step-by-Step Solution:

1. **Understanding the Assertion:**
- LASER (Light Amplification by Stimulated Emission of Radiation) is indeed used in various applications, including measuring distances to celestial objects like the moon. This is often done using laser ranging techniques, where a laser beam is directed towards the moon, and the time taken for the light to return is measured.

2. **Properties of LASER:**
- One of the key properties of laser light is that it can travel long distances with minimal loss of intensity. This is crucial for measuring large distances, such as the distance to the moon, because the signal must remain strong enough to be detected after reflecting off the moon's surface.

3. **Understanding the Reason:**
- The reason states that lasers are highly coherent sources of light. Coherence refers to the correlation between the phases of light waves at different points in space and time. In coherent light sources, such as lasers, the light waves have a constant phase relationship, which allows for better focus and less dispersion over long distances.

4. **Linking Assertion and Reason:**
- The coherence of laser light is indeed a significant factor that allows it to maintain its intensity over long distances. This property helps in accurately measuring the distance to the moon, as the reflected light can be detected with high precision.

5. **Conclusion:**
- Both the assertion and the reason are correct. The reason correctly explains why the assertion is true. Therefore, we can conclude that both statements are valid and related.

### Final Answer:
Both the assertion (A) and the reason (R) are correct, and the reason correctly explains the assertion.