You are working at NASA studying new life forms found in the Cueva de Villa Luz caves. You have collected an unknown specimen that appear to be 0.5 micron in size, have a cell membrane and ribosomes. This specimen could be ...

A bacteria

Explanation:

Cueva de Villa Luz is a cave in Mexico famous for high concentrations of hydrogen sulfide and sulfuric acid in water.

Organisms living in this conditions should be adapted. The only known organisms capable of obtaining energy from hydrogen sulfide (lithotrophs) are prokaryote.

We are also told that the specimen collected appears to be 0.5 micron, which is in accordance with bacteria. It has a cell membrane and ribosomes, which they do have as well.

This specimen could be a lithotoph bacteria.