The correct option is D Blue
Resolution of the microscope is inversely proportional to the wavelength of light used. Thus, the shortest wavelength of visible light will give the maximum resolution. Among yellow, green, red and blue light colour. Blue (500 nm) have the shortest wavelength so, it will give the best resolution.
Easy way to remember is to know that VIBGYOR - Violet, Indigo, Blue, Green, Yellow, Orange and Red, go in the direction of increasing frequency and thus decreasing wavelength. Thus near ultra violet light has the best resolution.