Hello!
To answer this question we need an understanding of prokaryotic cells.
Prokaryotic cells are unicellular and lack internal membranes and some organelles such as a nucleus. One of the major groups that has this type of cells is bacteria because they are quite simple.
Things like plants and humans have very complex cells that all together make up larger parts of a system.
So in this particular case, your answer would be, D. Lactobacillus
Lactobacillus is a type of friendly bacteria found in the body!