What is the smallest living thing in the following list: frog embryo, mitochondrion, prokaryote, virus, atom?

Prokaryote.

Explanation:

Prokaryotes are known as the smallest living thing, and bacteria are the best example of prokaryotes. Due to the absence of an internal membrane, these organisms lack a true nucleus and other organelles.

The smallest organism that is present on earth can be identified by their size, volume, mass, weight, and genome size, etc. Mycoplasma is known as the smallest prokaryotic organism on earth which is 0.15-0.35 micrometer in size.