statement I: The intensity of a plane progressive wave does not change with change in distance from the source.
Statement II: The wavefronts associated with a plane progressive wave are planar.

Options

A. Statement I is true, statement II is true and statement II is the correct explaination for statement I.
B. Statement I is true, statement II is true and statement II is NOT the correct explaination for statement I.
C. Statement I is true, statement II is false.
D. Statement I is false, statement II is true.

Explanation

To analyze the statements provided in the question, we will break down each statement and evaluate its validity step by step.

### Step 1: Evaluate Statement I
**Statement I:** The intensity of a plane progressive wave does not change with change in distance from the source.

1. **Understanding Intensity:** The intensity (I) of a wave is defined as the power (P) per unit area (A) through which the wave is passing. Mathematically, it is expressed as:
\[ I = \frac{P}{A} \]

2. **Area Consideration:** For a plane progressive wave, the wavefronts are planar, meaning they extend infinitely in two dimensions. Therefore, the area through which the power is distributed remains constant as the wave propagates.

3. **Power Consistency:** The power emitted by the source remains constant as the wave travels. Since the area does not change for a plane wave, the intensity remains constant.

**Conclusion for Statement I:** The intensity of a plane progressive wave does not change with distance from the source. Thus, Statement I is **True**.

### Step 2: Evaluate Statement II
**Statement II:** The wavefronts associated with a plane progressive wave are planar.

1. **Definition of Wavefronts:** A wavefront is defined as the locus of points that are in the same phase of vibration.

2. **Characteristics of Plane Waves:** In a plane progressive wave, the wavefronts are indeed planar. This means that all points on a given wavefront oscillate in unison, and the wavefronts are parallel planes.

3. **Visualizing Wavefronts:** If you visualize a plane wave, you can think of it as a series of flat surfaces (planes) that propagate through space. This is a characteristic feature of plane waves.

**Conclusion for Statement II:** The wavefronts associated with a plane progressive wave are planar. Thus, Statement II is **True**.

### Final Conclusion
Both statements are true. Moreover, Statement II serves as a correct explanation for Statement I, as the nature of the wavefronts (planar) directly supports the constancy of intensity with distance from the source.

### Summary
- Statement I: True
- Statement II: True
- Statement II explains Statement I.