Final answer:
The word that best completes the sentence is 'car' (because) as it provides a reason or explanation for vacuuming every day.
Step-by-step explanation:
In this sentence, the word that best completes it is 'car' (option C). 'Car' means 'because' in French, and it is used to give a reason or explanation for something. So, the complete sentence would be 'Je passe l'aspirateur tous les jours car j'ai une allergie à la poussière,' which translates to 'I vacuum every day because I have a dust allergy.'
Learn more about Completing sentences in French