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The sentence that is using a relative pronoun correctly is "When you've completed the assignment, please return it to Mr. Johnson or myself".
What is relative pronoun?
"A relative pronoun is a word that is used to connect an independent clause to a dependent clause."
The sentence that is using a relative pronoun correctly is "When you've completed the assignment, please return it to Mr. Johnson or myself." because the pronoun "when" is connecting an independent sentence that is "please return it to Mr. Johnson or myself" to a dependent sentence and also relating the both clauses by referring to a time .
To learn more about relative pronoun here
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