penicillin kills bacteria was discovered in 1928 by Alexander Fleming.

“Mold juice”, is the right answer.

Explanation:

Alexander Fleming was a microbiologist and physician from Scotland. He is popularly known for the discovery of the “Mold juice”. He found that it was not the mould but the juice that had killed the bacteria, therefore, he named it the mould juice penicillin. It was useful for diseases such as pneumonia, scarlet fever, meningitis, gonorrhoea and diphtheria etc. The penicillin was very popular during the Second World War as it was helpful to fight infections in soldiers.