Final answer:
The appropriate completion for the sentence is 'will have left', using the future perfect tense to refer to a finished action in the future. Option a is correct.
Step-by-step explanation:
The correct option that should fill in the blank in the sentence 'When you arrive, the plane __.' is 'will have left'. This sentence structure uses what is known as the 'future perfect' tense. The future perfect tense refers to a completed action in the future and is formulated as 'will have' + past participle of the verb. Therefore, in this case, 'the plane will have left' indicates that the plane's departure would be completed by the time you arrive.
Learn more about Future Perfect Tense