Which of the following colour of white light has the least wavelength?(a) red (b) orange (c) violet (d) blue

In the spectrum of white light, Red light has the longest wavelength and the least frequency (least deviation), whereas, violet light has the shortest or least wavelength and the highest frequency (maximum deviation), among all the colours in the visible spectrum.

As we go from red to violet (top to bottom) the wavelength keep on decreasing while the frequency keeps increasing. Thus, among the given colour of light, violet would have the least wavelength.