The correct answer is option 4) i.e. ground.Let's consider the usage of all the words given in the options: bailThe temporary release of an accused person awaiting trial, sometimes on a condition od lodging a sum of money as a guarantee for his appearance in the courtparoleThe temporary or permanent release of a person before the expiry of the sentence on the promise of a good behaviourguaranteeA formal assurance that a certain condition will be fulfilledgroundThe basis of release of some person before the expiry of his sentence. Important Points The sentence contains 'of good behaviour'. So the ground is the correct answer.If we give the answer as 'parole' it will be a repetition of the things.The completed sentence is: The prisoner was released on the ground of good behaviour.