Final answer:
The word in Spanish that best completes the sentence 'por favor' is 'Gracias' which means 'thank you'.
Step-by-step explanation:
The Spanish word that best completes the sentence 'por favor' is Gracias. This is because 'por favor' means 'please' in Spanish, and 'gracias' means 'thank you'. It is customary to say 'gracias' as a way to express gratitude or thankfulness after someone does something for you.
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