Microorganisms are microscopic living or infectious agents

Steps:

• Define microorganisms as entities too small to see without a microscope, including bacteria, fungi, and viruses.
• Identify bacteria as single-celled prokaryotes that are microscopic.
• Identify yeast as single-celled fungi that are also microscopic.
• Identify viruses as acellular infectious particles visible only under a microscope.

Why A is correct:

• By biological definition, microorganisms encompass bacteria (prokaryotes), yeast (unicellular fungi), and viruses (subcellular pathogens studied in microbiology).

Why the others are wrong:

• B excludes yeast, which are unicellular fungi and thus microorganisms.
• C includes only bacteria, omitting yeast and viruses as microorganisms.
• D excludes bacteria, which are prokaryotic microorganisms.

Final answer: A