The diffraction effect can be observed in:
$\left( A \right)$ only sound waves
$\left( B \right)$ only light waves
$\left( C \right)$ only ultrasonic waves
$\left( D \right)$ sound as well as light waves

Hint: The bending of light or waves around the corners of an obstacle into the region of geometrical shadow of the obstacle. The object diffracting effectively becomes a secondary source of the propagating wave. Using the above statement determines the answer to the above question.

Complete answer:
Bending of lights around edges when it passes around an edge or through a slit is called diffraction. Various phenomena occur when a wave encounters an obstacle.
Light or waves bending around the corners of an obstacle into the region of geometrical shadow of the obstacle. The object diffracting effectively becomes a secondary source of the propagating wave.
Oscillations can be parallel or perpendicular to the propagation of the wave or light direction. Geometrical orientation of the oscillation can be specified by the polarisation of waves.
The particles are vibrated in the same direction that the wave travels in the case of longitudinal waves. Transverse wave emerges from the polariser vibrating only in one plane.
When the distance between the fingers is very small then only the diffraction pattern occurs. It is a very important condition for the diffraction to occur. Slit width has to be comparatively less than the wavelength of light.
Hence diffraction can be observed both in sound and well as light.

Hence option $\left( D \right)$ is the correct option.

Note: Diffraction occurs prominently with longer wavelengths. The object diffracting effectively becomes a secondary source of the propagating wave. Diffraction depends on the slit size and the wavelength. Various phenomena occur when a wave encounters an obstacle.