Lets assume that intelligent extraterrestrials visited earth and gave a human a gift of allowing him/her to ask one question and just one. These beings come from far away, humans can't see their ships and don't know they exist and that they are already here.
You know this because you are the human that randomly was selected to receive that gift, you will not be able to see them nor hear their voices you can only write your question and they will look at it from the sky, you can leave it on an outdoor space, laying on the street. They will answer sending you a letter by mail.
The first condition is that you can only ask questions about them, the second condition is that you can't ask questions about the universe or how they got here or what are their intentions. You are allowed to ask about them and only them, the third condition is that you can't tell anyone about this and that you have one hour to ask. if any of these conditions are not met they wont answer you back.
The interesting part is that they will judge human kind with your question, that's why they chose a random average human, if your question makes sense for them they might make themselves appear so that all the world can see and know they do exist. They will always know if you are asking a question because you really want to know or just because you want to make the human kind seem better to them, so keep the answer true to yourself, something that you really want to know about them.
What would your question be and why?
I made this scenario because I wanted to know how many people would ask for help and how many would complicate a simple question.