Concept:The past perfect tense shows an action completed before another action in the past.
Explanation:The sentence says the lecturer entered the hall. This is a past action.
The cleaning of the board happened before the lecturer entered. Therefore, it is the earlier of two past actions.
To express the earlier action, we use the past perfect tense.
The past perfect tense is formed as:
had + past participle.
The past participle of "clean" is "cleaned". So we write "had cleaned".
Option A is present continuous, option B is present perfect, and option C is simple present. None of these fit because the time is in the past.
Only option D correctly expresses the completed action before another past event.
Answer:D. had cleaned