Answer: The sentence that correctly uses parallel structure is the third one: My best friend likes watching movies and playing basketball.
Step-by-step explanation: A parallel structure is a construction that repeats the same pattern of words or grammatical form. The third sentence ("My best friend likes watching movies and playing basketball") uses parallel structure correctly because it includes the-ing form of the verb twice after the verb "likes". In contrast, the second and fourth sentences mix the -ing form of the verb and the to-infinitive form, while the first sentence includes a noun and a verb in the -ing form. In that way, the third sentence is the only one that presents the same grammatical form twice and; therefore, the only one that uses parallel structure correctly.