All have beta lactam ring EXCEPT -
Ans. is 'b' i.e., Linezolid beta-lactam antibiotics contain beta-lactam ring. These are penicillins, cephalosporins (e.g. cefotoxime), monobactams (aztreonam) and carbapenems (imipenem, meropenem).
Penicillin
Linezolid
Cefotaxime
Imipenem