Here it is a question of situation.

If you ask about what this “monsieur” prefer at this moment, like you’re offering him some tee or coffee you’ll use “du”. “Préférez-vous de ça ou de ça” (You’re not giving him all the coffee on earth but just some of it)

If you ask him about his taste, you would use “Préférez-vous le thé ou le café, monsieur?”.

If I didn’t made myself clear enough. “du” is because you’re offering him some of it. “le” is more about the ‘concept’ or taste.

With some context, I bet someone is offering him some. If not, this is not correct or there is no context at all and you found this in an exercise.