In which context is the phrase "had decided" used correctly?

The phrase "had decided" is used correctly in the context of option D because it employs the past perfect tense, which is appropriate for conveying a completed action that occurred before another point in time in the past. In this case, the decision being referenced was made prior to the moment of seeing the evidence, establishing a clear chronological relationship between the two actions.

Using "had decided" indicates that the decision is not only in the past but is also a precursor to another past event (seeing the evidence). This structure helps clarify the timeline and is especially useful in narratives or discussions that involve multiple time frames. The other options do not maintain this clear sequence or use tenses appropriately, which could lead to confusion about when the decision was made relative to seeing the evidence.