Final answer:
The correct relative pronoun to complete the sentence is 'who', as it introduces a clause giving additional information about John Lennon, who co-founded the Beatles.
Step-by-step explanation:
The correct answer to the question, "John Lennon, __________ founded the Beatles with Paul McCartney in 1957, is remembered for his masterful songwriting ability." is option a. Who. The sentence requires a relative pronoun to introduce the clause that gives additional information about John Lennon. 'Who' is the appropriate relative pronoun because it refers to a person, in this case, John Lennon. Therefore, the sentence should read: "John Lennon, who founded the Beatles with Paul McCartney in 1957, is remembered for his masterful songwriting ability."