Final answer:
The correct indirect narration of the sentence is option a) The police inquired when the accident had taken place. This reflects the correct change in verb tense and sentence structure required for reporting questions indirectly in English grammar.
Step-by-step explanation:
The correct answer to the question of changing the direct speech, "The police said, 'When did that accident take place?'", into indirect narration is: a) The police inquired when the accident had taken place. This is because when reporting questions in indirect speech, the sentence structure changes from a question format to a statement. The auxiliary verb "did" is not used in the indirect statement, and the tense of the verb changes from the simple past "did take" to the past perfect "had taken" to indicate that the action happened at a time before the reporting verb (inquired).