Ozone is prepared by the action of silent electric discharge upon oxygen in an apparatus called ozonizer. The reaction

The reaction taking place for the formation of 3 O in the ozonizer is given below: Electric Discharge 2 3 3O 2O → The enthalpy of this reaction is H 248.5 Since the value of H is +Ve. So the reaction absorbs energy i.e. the reaction is endothermic. Hence, option (b) is the correct answer.