The correct option is D 100X
The most powerful lens of the light microscope is the 100X oil immersion objective because light is refracted every time it passes through a medium with a different refractive index, the quality of the image is reduced with each passage. Thus, by reducing the number of such passages to a minimum, the clarity, brilliance and resolving power is preserved. Immersion oil has been formulated so that it has a refractive index identical to that of glass. Thus, two changes in refractive index can be eliminated by placing a drop of immersion oil on the specimen, and immersing the 100X oil immersion objective directly into the drop.