The correct answer is lactic acid fermentation.
Lactic acid fermentation refers to a metabolic procedure by which glucose and other six-carbon sugars, like lactose or sucrose are transformed into cellular energy and the metabolite lactic acid that is a lactate in solution.
It is an anaerobic fermentation reaction, which takes place in certain kinds of bacteria and animal cells, like muscle cells.
One is immune system.
Cells are the most simple level of organization, as tissues contain cells, organs contain tissues and systems contain organs, so R would be your answer.
