Final answer:
The word that best fits the definition is 'delgado,' which means thin in English.
Step-by-step explanation:
Tío means uncle in Spanish. In the given question, '¿Tu tío es gordo?' translates to 'Is your uncle fat?' The response 'No, es delgado.' means 'No, he is thin.' So, the word that best fits the definition is 'delgado,' which means thin in English.
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