Final answer:
Harry moved into Dudley's second bedroom right after his parents' disappearance.
Step-by-step explanation:
The subject of this question is English in Middle School.
The correct answer to the question is option D: His parents' disappearance.
In the given reference, it is mentioned that Harry was delivered to the Dursleys' doorstep as an orphan. This implies that Harry's parents disappeared or passed away. Therefore, Harry moved into Dudley's second bedroom right after his parents' disappearance.