Microscopy

The resolving power of a microscope is its ability to show two close points as separate; it improves as the wavelength of the radiation used becomes shorter.
A light microscope uses visible light and can resolve points about 0.2 micrometre (200 nm) apart.
An electron microscope uses a beam of electrons, whose wavelength is much shorter than that of visible light, so it can resolve points only about 0.1 to 0.2 nm apart.
Its resolution is therefore much higher than that of the light microscope, which made it possible to see the fine structure (ultrastructure) of the cell and its organelles.
The statement giving 200-350 nm for the electron microscope has the two values interchanged.
Electrons have very poor penetrating power, so the electron microscope needs ultra-thin sections, and the wavelength of electrons is shorter, not longer, than that of light.
So the breakthrough came because the resolution power of the electron microscope is much higher than that of the light microscope.

The resolving power of a microscope is its ability to show two closely placed points as separate.
The limit of resolution is $d = \dfrac{0.61\lambda}{n\sin\theta}$ (Abbe's formula), so it is directly proportional to the wavelength $\lambda$ of the light used.
The shorter the wavelength, the smaller the distance that can be resolved, i.e. the better the resolution.
Among the given colours, blue light has the shortest wavelength (about 450 nm), compared with green (about 530 nm), yellow (about 580 nm) and red (about 700 nm).
The magnification of the eyepiece and objective does not change this.
So the object should be illuminated with blue light for the best resolution.