Final answer:
The correct word to fill in the blank is 'debo'. It is the first person singular form of the verb 'deber', meaning 'to owe' or 'to have to' in English.
Step-by-step explanation:
The correct word to fill in the blank is debo.
The full sentence would be: Yo debo libros.
The word debo is the first person singular form of the verb deber which means 'to owe' or 'to have to' in English.
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