Pasteur's study of fermentation was critical to the development of microbiology because a. fermentation chemistry occurs in the living body. b. it showed that microorganisms bring about chemical changes. c. it demonstrated that yeasts are involved in disease. d. the body undergoes rapid fermentation after death.

B. It showed that microorganisms bring about chemical changes

Explanation:

In 1857 Pasteur was working to solve a problem with the distillers of Lille in wich in their barrels the alcoholic fermentation become in a  lactic fermentation, and of course, that was something that they didn't want. Throughout diferents studyes Pasteur discover that different microorganism where involved directly (where responsible) in different types of fermentation. That was the key to start the microbiology studies worldwide.