Select the most appropriate option that can substitute the underlined segment in the given sentence. If there is no need to substitute it, select ‘No replacement required’.

If they would have departed earlier, they could have reached the venue before sunset.

The correct answer is If they had departed.

Explanation:
The given sentence represents a third conditional structure, which is used to describe a hypothetical situation in the past and its impossible outcome in the past.

The standard grammatical structure of a third conditional sentence is:
If + Past Perfect (had + V3), Subject + would/could/might + have + V3

In the original sentence:
- The 'if' clause incorrectly uses the modal past conditional "If they would have departed".
- The main clause correctly uses "they could have reached".

To fix the error, the conditional clause must be put into the past perfect tense ("had departed"). Therefore, "If they would have departed" should be substituted with "If they had departed".

Corrected Sentence:
"If they had departed earlier, they could have reached the venue before sunset."