What antibiotic was accidentally discovered by Sir Alexander Fleming when he noticed a lack of bacteria in a petri dish?

The antibiotic that Sir Alexander Fleming discovered is penicillin. This discovery occurred in 1928 when Fleming observed that a mold called Penicillium notatum contaminated one of his petri dishes, killing the surrounding bacteria. This lack of bacterial growth around the mold indicated that it was producing a substance effective against bacteria, which he later identified as penicillin. This accidental observation led to the development of penicillin as the first widely used antibiotic, revolutionizing the treatment of bacterial infections.

The other options listed are important medications but are not related to Fleming's discovery. Aspirin is a pain reliever and anti-inflammatory drug that was developed from salicylic acid. Tetracycline is a broad-spectrum antibiotic that was discovered later in the 1940s, and amoxicillin is a penicillin derivative that was developed after the introduction of penicillin into medical practice. Thus, the historical significance and context of penicillin make it the correct answer.