Final answer:
To replace the noun in the command 'Abre la puerta' with a pronoun, use 'la' to form the command 'Ábrela'.
Step-by-step explanation:
To replace the noun in the command 'Abre la puerta' with a pronoun, you would use 'la' as the pronoun. Therefore, the correct command form would be 'Ábrela'.
Learn more about Pronouns in Spanish