Thank you for the opportunity to help you with your question!
Let's break down each answer option.
If the gathering were political, like you were going over to the house to discuss a campaign strategy, then the phrase would be political. Clearly, the example says going over to a friend's house, so this scenario is unlikely.
If the gathering were economic, then the dinner would be over something financial. Maybe a banker is discussing a client's options over dinner. However, this is rarely done in a friend's home, in such an informal setting.
Finally, if the gathering were social, it would involve time and conversation that occurs on a day to day normal activities done between groups of people. Since we socialize, or meet up with, our friends, the gathering seems to best fit a social situation.
Please let me know if you need any clarification. I'm always happy to answer your questions.