Final answer:
Reactive-oxygen species are the most likely defense mechanism to protect plants from microbial pathogens.
Step-by-step explanation:
The defense mechanism that is most likely to protect a plant from a microbial pathogen is reactive-oxygen species.
Plants produce reactive-oxygen species, such as hydrogen peroxide and superoxide, as a response to microbial infections. These reactive-oxygen species can damage the pathogen's DNA and proteins, inhibiting their growth and replication.
Additionally, reactive-oxygen species can trigger the production of other defense mechanisms, such as antimicrobial proteins, to further protect the plant from the pathogen.