Which of these best demonstrates mutualism between certain types of bacteria and humans

I cannot find the propositions if it is a MCQ, so I will tell what a bacterial mutualism is:

Mutualism is an exchange between species (two or more) in which the mixed organisms profit from this exchange. Unlike commensalism, the associated species adapt, because the modification of one can affect the survival and reproduction of the affiliated species.

Many mutualistic bacteria live and develop in us, for example, the intestinal flora synthesizes vitamin K, a vitamin that is necessary for the absorption of our food. This same intestinal flora also hinders the proliferation of pathogenic bacteria as well as that of commensal bacteria potentially dangerous.

Bacteria and the human both get benefit from each other and this relation is mutualism. Bacteria stay inside nose, mouth, intestines, and many more areas of the body surface. They help human by giving response to pathogenic bacteria for protection of immune system.

They metabolize nutrients, vitamin production and wastes process in human’s digestive system. On the other hand, human provides them place to live. These above best demonstrates mutualism between bateria and humans.    

it's c. (to reduce the greenhouse effect/carbon gasses)