Final answer:
Scanning electron microscope is NOT useful for observing living cells because the vacuum environment and specimen preparation required for electron microscopy kills the cells, unlike light microscopy methods.
Step-by-step explanation:
The correct option is A. scanning electron microscope. Electron microscopes, such as the scanning electron microscope and the transmission electron microscope, require a vacuum environment and the specimen preparation process for viewing kills the cells.
Therefore, living cells cannot be observed using electron microscopy. In contrast, light microscopes like the phase-contrast microscope, brightfield microscope, and darkfield microscope enable the observation of living cells, as they do not require a vacuum and can provide contrast to distinguish cells without killing them.