Final answer:
The correct pronoun to fill in the blank is 'te'.
Step-by-step explanation:
The correct pronoun to fill in the blank is te. The sentence should be written as A ti te gustan las plantas. In Spanish, the pronoun 'te' is used to indicate the second person singular, which corresponds to 'you'. It is used as an indirect object pronoun after the preposition 'a' and before the verb 'gustan'.
Learn more about Spanish pronouns