Diffraction is the change in a wave's direction as it travels around barriers. Diffraction is observed in both light and sound waves. This means that options 1 and 2 are wrong.
For any opening or slit, waves with longer wavelengths bend more than waves with shorter wavelengths. If the wavelength is smaller than the opening, it would be difficult for diffraction to occur. Thus, the correct option is
It occurs best when the slit width is less than the wavelength of a wave.
