The drug inactivated in plasma by spontaneous non enzymatic degradation is:
Atracurium in plasma undergoes spontaneous non enzymatic degradation(Hoffman elimination)in addition to that by cholinesterases. All other given drugs don't undergo Hoffman elimination. From Tripati 7th edition Page no 353
Atracurium
Vecuronium
pipecuronium
Pancuronium
Anesthesia