Answer: Rinsing with alcohol during the washing step stripped the bacteria off the glass slide.
Explanation:
The step that is being wrong in the whole experiment is that the student is washing the bacteria on the slide with alcohol.
Alcohol might have washed off all the bacteria as it is corrosive in nature. The bacteria that is going to be observed in the experiment is being killed when it is washed off.
This was the wrong step opted by the student if he finds any alternative solution to this then the bacteria can be observed in the microscope.