문법 문제에서 that이 틀린 이유가 궁금해요

문장에서 어법상 틀린 부분을 찾는 문제예요.
After moving to a new city, I joined the company baseball team. Being the oldest player, I had to play the outfield. During one game, I made a few mistakes. Then I kept hearing someone shouting, “Way to go, Mr. Green” and “You can do it, Mr. Green.” I was amazed someone would know my name in this strange city. After the game, I met my wife and son and asked that they knew who was shouting encouragement in the stand. My son spoke up and said, “Dad, it was me.” I asked why he was calling me Mr. Green and he replied, “I didn’t want anyone to know that I’m your son.”
④번 that이 정답이라는데 왜 틀린 건지 설명 부탁드려요.