Option 1: Penicillin is a widely used antibiotic that is effective against many bacterial infections. However, it is not considered a drug of last resort. There are other antibiotics available that may be used when penicillin is not effective.

Option 2: Tetracycline is another commonly used antibiotic. It is effective against a wide range of bacterial infections, but it is not considered a drug of last resort.

Option 3: Chloramphenicol is considered a drug of last resort for human beings. It is a broad-spectrum antibiotic that is effective against many types of bacteria. It is usually reserved for treating infections that are resistant to other antibiotics.

Option 4: Streptomycin is an antibiotic that is primarily used to treat tuberculosis. While it is an important antibiotic for tuberculosis treatment, it is not considered a drug of last resort for all types of bacterial infections.

In conclusion, the correct answer is option 3, Chloramphenicol, as it is considered a drug of last resort for human beings. It is important to note that the use of this drug is typically reserved for severe infections that are unresponsive to other antibiotics.