All of the reasons except Business and Study are clearly and obviously not applicable to your case. Let's ignore them.

Is your main reason for attending to present, or to attend? If you're there to present, saying "study" is deceptive - you aren't there to learn anything. If you were already graduated, and employed (in academia or in private industry) I think you would check Business. Money doesn't have to change hands for something to be a business trip. For example, when I travel to attend a conference and learn about new things, it's a business trip for me. When I travel to meet people and discuss ways my company can help them in the figure, it's business even though I don't charge for that conversation. And so on.

It's not like this single word is the only explanation you are able to provide, is it? You're going to be asked for a little more detail, surely? So go with Business, and explain what you're doing and why.

All of the reasons except Business and Study are clearly and obviously not applicable to your case. Let's ignore them.

Is your main reason for attending to present, or to attend? If you're there to present, saying "study" is deceptive - you aren't there to learn anything. If you were already graduated, and employed (in academia or in private industry) I think you would check Business. Money doesn't have to change hands for something to be a business trip. For example, when I travel to attend a conference and learn about new things, it's a business trip for me. When I travel to meet people and discuss ways my company can help them in the future, it's business even though I don't charge for that conversation. And so on.

It's not like this single word is the only explanation you are able to provide, is it? You're going to be asked for a little more detail, surely? So go with Business, and explain what you're doing and why.