The participants ___ working on the complex project for one hour when he suddenly ___ the room. ?

The sentence in question describes two actions occurring in sequence, which necessitates careful attention to the verb tenses used. The phrase "The participants ___ working on the complex project for one hour" indicates an action that was ongoing in the past. The correct choice here is "had been," which is the past perfect continuous tense. This tense emphasizes the duration of an activity that was happening before another past event occurred.

In contrast, the phrase "when he suddenly ___ the room" indicates a completed action in the past that interrupts the ongoing activity. The use of "entered" fits perfectly here, as it aligns with the moment the action of entering happens abruptly during the preceding activity.

Thus, option D formulates the sentence correctly as: "The participants had been working on the complex project for one hour when he suddenly entered the room." This structure clearly shows that the participants' work was happening continuously up until the moment the person entered, demonstrating the relationship between the two actions in the past. Other options either do not provide the correct tense or fail to accurately convey the sequence of events.