Final answer:
To substitute the direct object in the sentence 'Comi el plátano,' use the pronoun 'lo.'
Step-by-step explanation:
To substitute the direct object in the sentence 'Comi el plátano,' you would use the pronoun 'lo' because 'plátano' is a masculine singular noun. Therefore, the correct substitution is 'Comí lo.'
Learn more about Spanish pronouns