Question

under which objective lens is the fine adjustment knob used?
low power objective lens only
low and high power objective lens only
it can be used for all three objective lenses, but in low power it is used after the coarse adjustment knob

Explanation:

The fine - adjustment knob can be used with all objective lenses on a microscope. With low - power lenses, it is used after the coarse adjustment knob to get a sharp image. With high - power lenses, only the fine adjustment knob should be used to avoid crashing the lens into the slide.

Answer:

It can be used for all three objective lenses, but in low power it is used after the coarse adjustment knob