MAIN clause'.If 'if clause' is in the PAST perfect tense 'would have+ V3' is used in the 'main clause'.In the given sentence 'would have+ V3' has been used in the main clause, THEREFORE, past perfect tense should be used in the 'if clause'.'had+ V3' i.e. 'had+ been' is mentioned only in option 2.Therefore, the correct sentence is 'If he had been more CAREFUL, he wouldn’t have had an accident. '.