By the time we . . . . . . . . our preparations for the congress, most of the participants. . . . . . . . at the hall.

Answer: Option D

The correct sequence is "finished / had arrived".

Explanation:

This sentence involves two actions in the past, where one action was completed before the other.

Past Perfect Tense ("had arrived") is used to describe the action that happened first — participants arriving.

Simple Past Tense ("finished") is used for the action that happened later — we finished preparations.

The sentence means that by the time we completed our preparations, most participants had already arrived.

Incorrect Options:

Option A: have finished / had arrived – "Have finished" is present perfect, which doesn’t fit with the past perfect in the second clause.

Option B: will have finished / arrive – This uses future perfect and present, which doesn't match the past context.

Option C: finished / have arrived – Mixing simple past with present perfect is not appropriate for this timeline.

Option E: finish / would arrive – This suggests a hypothetical or future-in-the-past situation, which is not the intended meaning.

Therefore, Option D is grammatically and logically correct.