Answer : 1. "will have finished"
Explanation :

The phrase "By the time he arrives" indicates a point in the future relative to another action. When we use this phrase, we typically use a future perfect tense to indicate an action that will be completed before another future action.

• a) will have finished - This option correctly uses the future perfect tense ("will have finished"), indicating that the action of finishing dinner will be completed before the arrival of "he".
• b) have finished - This is in the present perfect tense, which suggests that the action of finishing dinner has already been completed, without necessarily linking it to the future arrival.
• c) will be finishing - This uses the future continuous tense, which implies that the action of finishing dinner will be ongoing at the time "he" arrives, rather than already completed.
• d) finished - This is in the simple past tense, which does not fit the context of a future action ("By the time he arrives").

Therefore, the correct answer is a) will have finished.

This choice correctly indicates that the action of finishing dinner will be completed by the time "he" arrives in the future.