A. The paramecium is not very small It can easily be observed using the light
microscope.
B. You want to observe life processes. You cannot use an electron microscope  to observe living specimens.
Explanation:
The slipper and elongated shape of the paramecium is visible to the naked eye. It is easily seen under the light microscope.
Electron microscope uses a beam of electrons and not beam of light as in light microscope. Living cells cannot be observed under electron microscope because of the use of vaccum in which samples are placed. Vacuum is used because the electron beam moves better in it.
Live samples are rarely stained as compared to non-living samples.
Since students intend to observe how paramecium injects food, it is advised to use a light microscope.