By the time we arrive, everyone else _____

The sentence implies that when "we" arrive, a certain action will have occurred prior to that moment. The phrase "by the time we arrive" suggests a future point in time. Therefore, the correct choice conveys an action that is expected to be completed before that specified future moment.

In this case, "will have left" indicates that everyone else will have completed the action of leaving before our arrival. This construction utilizes the future perfect tense, which is ideal for expressing that one action (leaving) will occur before another action (arriving).

The other options either denote actions that do not align correctly with the intended future context or indicate a different timing that does not fit the structure of the sentence. This makes "will have left" the only answer that accurately captures the future sequence of events relative to the arrival.