Which was the first discovered antibiotic?

The correct answer is Option (3) → Penicillin

Option (3) Penicillin — Correct. Penicillin was the first antibiotic to be discovered. Alexander Fleming found it while working on Staphylococci; Chain and Florey later established its full value.

NCERT Extract:
Antibiotics – “… You are familiar with the commonly used antibiotic Penicillin. … 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. However, its full potential as an effective antibiotic was established much later by Ernest Chain and Howard Florey.”