Answer;
Rotating the coarse adjustment knob.
A microscope slide viewed with high power can most likely be damaged by rotating the coarse adjustment knob.
Explanation;
The coarse adjustment knob moves the stage up and down for focusing.
When using a microscope one should use the coarse adjustment and the low power objective to locate and observe a specimen on a slide.
The coarse adjustment of a microscope should be used with the low-power objective, but not with the high power objective.