What was Alexander Fleming working on when he discovered Penicillin?

The correct answer is Option (3) → Staphylococci bacteria

Penicillin, the first antibiotic, was a chance discovery. Alexander Fleming was working on Staphylococci bacteria when he saw a mould growing in one of his unwashed culture plates. Around the mould the Staphylococci could not grow. He found that a chemical made by the mould was responsible and named it Penicillin, after the mould Penicillium notatum.

NCERT Extract:
Antibiotics – “… Do you know that Penicillin was the first antibiotic to be discovered, and it was a chance discovery? Alexander Fleming while working on Staphylococci bacteria, once observed a mould growing in one of his unwashed culture plates around which Staphylococci could not grow. He found out that it was due to a chemical produced by the mould and he named it Penicillin after the mould Penicillium notatum.”