The given answer "Good afternoon" is the correct one because it is a common greeting used during the afternoon hours. It is a polite and friendly way to acknowledge someone during this time of the day. The other options, "こんにちは" means "Hello" in Japanese, "Excuse me" is used to get someone's attention or ask for forgiveness, and "Let's eat" is a suggestion to have a meal together. Therefore, "Good afternoon" is the most appropriate response in this context.