Du refers only to one person, so right now I would use "Du spørger et spørgsmål" (You are asking a question, talking to you and only you)
Dig is used for the same but when you are the object of the sentence, so I would say "Jeg svarer dig" (I'm answering you).
I is used for speaking to more than one person (like the souther American "y'all". I har interessante spørgsmål is "You (all) have interesting questions).
Jer is used for the same, but when they're the object of a sentence. "Jeg snakker til jer" is "I'm talkin to you (all)"