Final answer:
The word that best completes the sentence is 'tío', which means 'uncle' in Spanish.
Step-by-step explanation:
The word that best completes the sentence is tío. In Spanish, 'tío' means 'uncle'. The sentence is asking for the word that describes the brother of the student's mother. Since the word 'tío' means uncle, it fits perfectly in the given sentence.
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