Final answer:
The correct word to use in the blank is 'pasaban'.
Step-by-step explanation:
The correct word to use in the blank in the sentence is pasaban. The sentence is talking about our grandparents returning home happily after spending time with us. The verb pasaban is the correct conjugation of the verb pasar (to spend time). In this context, it agrees with nuestros abuelos (our grandparents).
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