You are going to view your Staphylococcus aureus slide under oil immersion. You see small spots as you are focusing on the slide. How can you distinguish small spots that are part of the specimen on the slide from specks of dirt on the microscope lenses ?

This can be achieved by increasing the  resolution  of the objective lens of the microscope. To achieve this both the objective lens  ( with the dirt)and the specimen   are immerse in the oil.   The type of objective lens used  for the immersion is called optical immersion objective

The oil is first applied to the specimen mounted on the microscope stage.Thus by raising the stage to torch  objective lens both the lens and the specimen are immerse in oil. The oil  Increases the resolution of the objective lens.

Generally the high refractive indices of  the oil  and water is close. Therefore  the oil  increase  angular range  at which  the incident light on  the lens  will be transmitted , and therefore. focus on the specimen. Thus the oil with  the objective lens reduces the refraction of light so that as light  incident on the specimen  the high refractive index  of the lens and oil reduces the diffraction of light, and therefore focusing on the specimen increase.

Consequently, by adjusting the focal length , the objective  lens at high magnification is able to resolve the dirt on it and the spots on the specimen. That is   distinguished between the dirt on it and  spots on the specimen as two distinct images rather than one.

as many times as you want

c should be the answer