To observe diffraction, the size of the obstacle:
A) Has no relation to wavelength.
B) Should be $\lambda /2$ where $\lambda $ is the wavelength.
C) Should be much larger than the wavelength.
D) Should be of the order of wavelength.

Hint: When the wave of light passes through the particles of the obstacle object and due to the sudden change in densities, the speed of light changes which give birth to the phenomenon of diffraction. Use the basic knowledge of diffraction and apply it to reality to answer this question.

Complete answer:
We know that diffraction takes place when a wave of light passes through the particles of an obstacle object whose density is different from that of the medium in which light was moving previously, and hence the speed of light changes which result in diffraction. Using this definition and the basic concepts of diffraction, we will solve the question by looking at the options one by one.
Option A: Has no relation to wavelength
This option is obviously incorrect as wavelength is one of the main deciding components about the extent of diffraction in a medium.

Option B: Should be $\lambda /2$ where $\lambda $ is the wavelength
If the wavelength is equal to $\lambda $ , then $\lambda /2$ the size of the obstacle particle will be too small for an observer to observe diffraction through it.
This option is also incorrect.
Option C: Should be much larger than the wavelength
If the size of the obstacle particle is much, much larger than the wavelength, then the diffraction won’t take place at all. This can be considered as a law for diffraction.
Hence, this option is also incorrect.

Option D: Should be of the order of the wavelength
If the size of the obstacle particle is of the order of the wavelength, diffraction will take place and will be easily observable by the observer.
Hence, option D is the correct answer.

Note: Diffraction is a phenomenon which takes place under some specific conditions. However, these conditions are quite vast and it appears like diffraction can take place under any circumstances, but it is not the truth. Like in option C, diffraction won’t take place if the size of the obstacle particle is much, much larger than the wavelength of the ray of light.