Final answer:
Wax is not used to embed tissues for electron microscopy because it cannot be sliced thinly enough for electron beam penetration, which is essential for the high resolution that electron microscopy provides. Therefore, the correct option is a.
Step-by-step explanation:
The reason wax is not used to embed tissues for electron microscopy is primarily because wax cannot be sliced thin enough for the electron beam to penetrate. Electron microscopy requires specimens to be extremely thin, on the scale of 20-100 nanometers thick, to allow electrons to pass through and provide the necessary image resolution. In contrast, wax embedding is typically used for light microscopy where the visualization does not require such thin sections. Moreover, the process of dehydration and resin infiltration ensures that the specimen is sufficiently prepared to withstand the vacuum environment of the electron microscope, which would not be the case with wax.