The following sentence has been divided into parts. One of them may contain an error. Select the part that contains the error from the given options. If you don’t find any error, mark ‘No error’ as your answer.
The students were diligently / studying for their exams / for weeks before you arrived.

Correct option is D

We use past perfect continuous for the earlier, longer action and simple past for the later action.
Hence:
"had been studying" … before "you arrived."
Grammatical rule used:
Use past perfect continuous (had been + verb-ing) for an action that was ongoing over a period before another past action.
The past perfect continuous shows duration and completion of an earlier past activity.
The simple past is used for the later action.
Example:
He had been working at the company for 10 years before he retired.
She had been reading all day before the guests came.
Information Booster:
If the action is interrupted or leads up to another specific past point, use past perfect continuous.
If you simply want to describe what was happening at a particular time in the past, use past continuous instead.