This question tests the understanding of the past perfect tense, which is used to describe an action that was completed before another action in the past. The correct answer is B, 'He had left the office when I arrived,' because it correctly uses the past perfect tense ('had left') to indicate that the action of leaving the office was completed before the action of arriving. Options A, C, and D are incorrect because they use other tenses (present perfect, future perfect, and future, respectively) that do not fit the context of describing an action completed before another past action.