Which of the following color has the least wavelength?

Violet has the least wavelength.

In the visible spectrum of sunlight, violet has the shortest wavelength among the visible colors. It has a wavelength ranging from approximately 400 nanometers, making it the color with the least wavelength in the visible spectrum.
Whereas Red light has the longest wavelength in the VIBGYOR series. VIBGYOR is a combination of seven colors of light to form white light. It is known as the spectrum. 

Given lights in ascending order of their wavelength are as follows - $\mathrm{Violet}<\mathrm{ Blue}<\mathrm{ Orange}<\mathrm{ Red}.$

Hence, "violet" has the least wavelength.