Final answer:
Penicillin G is not suitable for milking cows but can be used in beef cattle. It is effective against gram-positive bacteria and cannot be administered orally. In contrast, penicillin V, ampicillin, and amoxicillin are acid stable and can be used in beef cattle orally.
Step-by-step explanation:
Penicillin G is a narrow-spectrum antibiotic that is given intravenously or intramuscularly. It is primarily effective against gram-positive bacteria. It cannot be administered orally because it is unstable in the acidic pH of the stomach. In contrast, penicillin V, ampicillin, and amoxicillin are acid stable and can be used in beef cattle orally.