The correct answer is option 'A' - plane.

Explanation:
To understand why the shape of the wave front of light from a distant star intercepted by the Earth is a plane, we need to consider the properties of light waves and how they propagate.

1. Nature of light waves:
Light waves are electromagnetic waves that travel in a straight line from their source. They have properties such as wavelength, frequency, and velocity.

2. Propagation of light waves:
Light waves propagate in all directions from their source, forming spherical wave fronts. These wave fronts can be imagined as concentric spheres expanding outward from the source.

3. Interception by the Earth:
When light from a distant star reaches the Earth, it is intercepted by the Earth's surface. At this point, the wave fronts of the light become distorted due to the irregular shape of the Earth's surface.

4. Wave front shape:
While the wave fronts of the light from a distant star are initially spherical, they become progressively flatter as they approach the Earth's surface. This is because the Earth's surface acts as a flat plane relative to the scale of the incoming light waves.

5. Plane wave front:
As the light waves approach the Earth's surface, the curvature of the wave fronts becomes negligible compared to the size of the Earth. Therefore, the shape of the wave front of the light from a distant star intercepted by the Earth is effectively a plane.

This phenomenon can be observed when starlight reaches the Earth's atmosphere and appears as a flat wave front. This is why telescopes are designed with flat mirrors and lenses to capture and focus the incoming starlight onto a detector or observer.

In conclusion, the shape of the wave front of light from a distant star intercepted by the Earth is a plane due to the negligible curvature of the wave fronts compared to the size of the Earth.