Explanation

Penicillin is a beta-lactam antibiotic that binds transpeptidase enzymes (penicillin-binding proteins), blocking the cross-linking of peptidoglycan in the bacterial cell wall; the weakened wall then ruptures. It does not target bacterial protein synthesis (the target of tetracyclines and macrolides), RNA synthesis (rifampicin), or DNA synthesis (quinolones). Hence A.