The correct option is A bacterial
Penicillin is an antibiotic used in treating bacterial infections. It was discovered by Dr Alexender Fleming when he was working on a culture of disease-causing bacteria. The growth of blue-green moulds was observed in one of his culture plates. On further observation, he found that there was no bacterial growth near the moulds, and these moulds had killed a significant part of the bacterial culture. The isolation and study of these moulds led to the discovery of penicillin.