**Diffraction of Electromagnetic Waves**

The diffraction of electromagnetic waves refers to the bending of the waves as they encounter an obstacle or pass through an opening. This phenomenon occurs when the size of the obstacle or opening is comparable to the wavelength of the waves. Diffraction is a fundamental characteristic of wave behavior and is observed in various fields such as optics, acoustics, and radio waves.

**Explanation of Correct Answer (Option D)**

Diffraction may occur around the edge of a sharp obstacle. When an electromagnetic wave encounters an obstacle with a sharp edge, such as a corner or a narrow slit, it undergoes diffraction. The wavefronts of the incident waves are disrupted and change direction as they pass around the edge of the obstacle. This bending or spreading out of the waves is known as diffraction.

**Principle of Huygens-Fresnel**

The principle of Huygens-Fresnel helps explain the phenomenon of diffraction. According to this principle, every point on a wavefront can be considered as a source of secondary wavelets. These secondary wavelets propagate in all directions, and the resulting wavefront is the sum of all these secondary wavelets. When an obstruction is encountered, the secondary wavelets from different parts of the wavefront interfere with each other, leading to diffraction.

**Effects of Diffraction**

Diffraction causes several observable effects, including:

1. **Bending of Waves**: As the waves encounter an obstacle, they bend around the edges, spreading out into the region behind the obstacle.

2. **Wavefront Distortion**: The wavefronts become curved, and the shape of the wave is altered due to interference between the secondary wavelets.

3. **Shadow Formation**: Diffraction can result in the formation of a shadow region behind the obstacle where the wave intensity is reduced.

4. **Fringe Patterns**: In some cases, such as when waves pass through a narrow slit, interference patterns known as fringes can be observed. These patterns arise due to the constructive and destructive interference of the diffracted waves.

**Conclusion**

In summary, diffraction of electromagnetic waves occurs when they encounter an obstacle or pass through an opening. It is characterized by the bending, spreading, and interference of the waves. While diffraction can occur in various scenarios, it is particularly pronounced around the edge of a sharp obstacle. This phenomenon is explained by the principle of Huygens-Fresnel, which considers each point on a wavefront as a source of secondary wavelets. Diffraction is a fundamental aspect of wave behavior and plays a crucial role in various applications, including optics, acoustics, and radio communications.