Answer : 2. "looking for"
Explanation :

• Looking at: This means to direct your gaze towards something, usually with interest or intent to see details. Example: "He was looking at the beautiful painting."
• Looking for: This means to search for something that is lost or needed. Example: "He is looking for his lost keys."
• Looking after: This means to take care of or be responsible for someone or something. Example: "She is looking after her younger sister."
• Looking on: This means to watch an event without becoming involved in it. Example: "They were just looking on as the argument continued."

In the context of the sentence "He is _______ his lost keys," the correct phrasal verb is "looking for," indicating that he is actively searching for his lost keys.