What action had taken place by the time the neighbor returned home?

The correct answer indicates that an action was completed prior to another event happening, which is reflected in the phrase “had already left.” This construction uses the past perfect tense, which is specifically designed to show that one action occurred before another action in the past.

In this context, saying the neighbor “had already left” means that the leaving happened before the neighbor returned home. This chronological order is key to understanding the situation: the action of leaving is complete by the time the neighbor arrives back.

The other choices do not convey this back-in-time context effectively. "Will leave" implies a future action that has not yet occurred. "Were arriving" suggests a simultaneous action happening in the past but does not indicate that the action was completed before the neighbor’s return. "Had just arrived" would imply that the neighbor was returning to find someone who had arrived, which doesn’t fit the timing suggested by the question.