Final answer:
The Spanish word that best completes the sentence is 'la sobrina', which means 'the niece' in English.
Step-by-step explanation:
The Spanish word that best completes the sentence is la sobrina. This word means 'the niece' in English. 'Luisa es la sobrina de Nelly' translates to 'Luisa is Nelly's niece.' It is important to match the gender and number of the noun with the possessive pronoun, which in this case is 'la' for a female singular noun.
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