Final answer:
The Spanish word that best completes the sentence 'El caballo corre muy rápido' is 'El caballo'. It means 'The horse' in English.
Step-by-step explanation:
The Spanish word that best completes the sentence 'El caballo corre muy rápido' is El caballo. This means 'The horse' in English. In the given sentence, 'corre muy rápido' means 'runs very fast', so the subject that matches that description is the horse.
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