Harry Potter formally meets Draco Malfoy first at Madam Malkin's Robes Shop in Diagon Alley, before meeting any Hogwarts students on the train. This occurs while Harry is being fitted for his school robes. Option D is correct. Options A, B, and C are incorrect because Harry meets Neville, Hermione, and Ron later on the Hogwarts Express. The encounter with Draco is significant because it establishes their rivalry immediately.