Which branch of science focuses on the study of microscopic organisms such as viruses, bacteria, protozoa, parasites, and some fungi and algae?

The correct answer is microbiology.

Microbiology is the branch of science that study microorganism. It contains a few sub-disciplines including virology (study viruses), parasitology (study parasites), mycology (study fungi) and bacteriology (study bacteria). Studied microorganism might be unicellular (one cell), multicellular (cell colony), or acellular (without cells).