Answer:B. Protein denaturation.
Explanation:The mechanism of action of boiling is primarily B. Protein denaturation.
Boiling works by causing proteins in microorganisms to denature, which means they lose their three-dimensional structure and function, leading to the death of the microorganisms. While high temperatures can also damage cell membranes and DNA to some extent, the primary and most effective action of boiling is denaturation of proteins, which disrupts the normal functioning of microbial cells.