Caroline turns her humanity back on in The Vampire Diaries in Season 4, Episode 6. This moment is significant in showcasing her character development as she struggles to reconnect with her emotions and her human side after being a vampire with her humanity switched off.

Additionally, there are references to her humanity switch being a recurring theme, with some scenes in later seasons (like Season 6, Episode 19) where Caroline interacts with this aspect of herself. But the initial key moment when she consciously turns her humanity back on is in Season 4, Episode 6.