Assertion: X− rays are used for studying the structure of crystals.
Reason: The distance between the atoms of crystals is of the order of wavelength of X− rays.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C. Assertion is correct but Reason is incorrect.
D. Both Assertion and Reason are incorrect.

Hint: Here, we have knowledge of the electromagnetic spectrum in addition to the wavelength concept, which allows us to visualise the spectrum in order to fully understand the speed of light concept. Determine the X-rays' wavelength as well.

Complete step by step solution:
The distribution of electromagnetic radiation according to energy, or alternatively according to frequency or wavelength, is known as the electromagnetic spectrum. As a reference of electromagnetic radiation, we can describe $X$-Rays or$X$-radiation. They are electromagnetic energy waves that are inert. The majority of them have a wavelength between 0.01 and 10 nanometres.

Violet has the shortest wavelength, while red has the longest. When all the waves are observed at once, they emit white light. UV light has a wavelength that is shorter than visible light and falls between the range of 10 nm to 400 nm, but longer than $X$-rays.

A property of the substance under study is its X-ray diffraction pattern. The percentage of crystallinity in natural fibres both before and after physical or chemical treatment can be found using the X-ray diffraction technique. Crystals have atoms spaced apart on the order of an X-ray wavelength.

They are diffracted when they touch a crystal. Studying the diffraction pattern is beneficial for understanding crystal structure. Therefore, we can say that both assertion and reason are correct and reason is the correct explanation for assertion.

Hence option A is correct.

Note:X-ray diffraction is a common method for determining the crystallinity and structure of solid materials. In summary, crystal X-ray diffraction is caused by a scattering process in which X-rays are scattered by electrons of atoms existing in the sample without affecting the wavelength. When X-ray contacts a metal surface with angle of incidence $\theta$ then it is reflected by the crystal’s atom at the same angle $\theta$.