Final answer:
Option A) Leeuwenhoek did not observe viruses as they are much smaller than the microorganisms he could see with his simple microscopes.
Step-by-step explanation:
The type of microbe that was NOT observed by Leeuwenhoek is a virus. Antonie van Leeuwenhoek is credited with being the first to observe microscopic life, including bacteria (which are prokaryotes), protozoa, algae, and single-celled eukaryotes like fungi, using his simple microscopes. However, viruses are much smaller than the bacteria and microorganisms Leeuwenhoek observed, and they were not discovered until much later with the advent of electron microscopes. Among the options listed, viruses are the only type that Leeuwenhoek could not have observed due to their minuscule size.