The sentence structure "No sooner... than" is used to express that one action or event happens immediately after another. In this case, we want to convey that the rain started immediately after we set out. A. did we set out when the rain had started: The use of "did we set out" is not the most suitable structure for expressing the immediate sequence of events. Also, the use of "had started" implies a past perfect tense, which doesn't align with the intended meaning. B. had we set out than the rain started: This option correctly uses the structure "No sooner... than," indicating the immediate succession of events. It is the most appropriate choice. C. were we setting out than the rain started: While this option is grammatically correct, it doesn't use the past perfect tense, and the use of "were we setting out" sounds less natural in this context. D. we had set out when the rain started: This option uses "when" instead of "than," and the use of "had set out" implies a past perfect tense, which doesn't precisely convey the idea of immediate succession. Therefore, option B, "No sooner had we set out than the rain started," is the most suitable choice as it accurately expresses the sequence of events with the appropriate structure.