Final answer:
The word that best completes the sentence is 'el cuñado'.
Step-by-step explanation:
The correct word to complete the sentence is 'el cuñado'. When translated to English, the sentence reads 'Luisa is the sister-in-law of Nelly.' In Spanish, 'el cuñado' means 'the brother-in-law', and since Luisa is referred to using a female name, the word 'cuñada' should be used, which is the feminine form of 'cuñado'.
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