The correct answer is: B. Went/was doing
The warden went into the room to see what the students were doing. The simple past tense is used to describe a completed action in the past. The past progressive tense is used to describe an action that was in progress at a specific time in the past. In this case, the warden went into the room at a specific time in the past (when the students were doing something) to see what they were doing.
A. Go/had been doing is incorrect because the simple past tense cannot be used with the past perfect progressive tense.
C. Gone/had been doing is incorrect because the past participle “gone” cannot be used with the past perfect progressive tense.
D. Went/were doing is incorrect because the simple past tense cannot be used with the past progressive tense.