Final answer:
The correct Spanish word to complete the sentence is 'piernas', which means 'legs' in English.
Step-by-step explanation:
The correct answer is piernas. In the sentence, 'Las personas tienen dos piernas,' the word 'piernas' means 'legs' in English. This sentence is describing how many legs people have, stating that they have two legs. Other options like 'pelos' (hair), 'cuellos' (necks), and 'narices' (noses) are not correct because they are not related to the number of body parts people have.
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