Question

which part of the microscope should a student use to focus while using high power?

Explanation:

When using high - power on a microscope, the fine - adjustment knob is used to focus. It makes small, precise adjustments to the focus. The coarse - adjustment knob is used for initial focusing at low power.

Answer:

The fine - adjustment knob (assuming one of the labeled parts is the fine - adjustment knob). Since the question is incomplete without knowing what parts P and Q are, a general answer is that the fine - adjustment knob is used for focusing at high power.