The correct answer is option 3: Penicillin. Penicillin is considered the first antibiotic, discovered by Alexander Fleming in 1928. Fleming accidentally observed that a mold called Penicillium notatum had the ability to kill bacteria. This discovery led to the development of penicillin as an effective treatment for bacterial infections.

Option 1: Terramycin is an antibiotic that was discovered in the 1950s, after penicillin had already been in use.

Option 2: Neomycin is an antibiotic that was discovered in 1949 and is commonly used in topical medications.

Option 4: Streptomycin is an antibiotic that was discovered in 1943 and was the first effective treatment for tuberculosis.

Therefore, option 3: Penicillin is the correct answer as it was the first antibiotic discovered by Alexander Fleming.