Question

penicillin is a common antibiotic. what is an antibiotic?
a substance that inhibits the growth of bacteria
a substance that only kills viruses
a substance that kills fungal infections

Explanation:

Antibiotics are substances that can inhibit the growth of bacteria or kill bacteria. They are not effective against viruses (ruling out the virus - killing option) and substances that kill fungal infections are antifungals, not antibiotics.

Answer:

a substance that inhibits the growth of bacteria