Answer:
See the answer below
Step-by-step explanation:
The slide could have broken due to the ramming of the objective (especially the high power objectives) into the slide on the stage of the microscope while trying to bring the object on the slide into focus.
It is recommended to start with the lowest objective while trying to focus a slide. Thereafter, the next higher objective can be switched to and the image brought into focus once again. This can be repeated until the desired magnification of the image is reached.
However, at higher objective powers, the coarse adjustment knob should be avoided to avoid the objectives touching/breaking the slide. Instead, the fine adjustment knob should be used.
Hence, the breaking of the slide in the illustration could have been due to the use of the coarse adjustment knob at higher objective powers and the ramming of the objective into the slide.