Which of the following non-manual markers indicate a yes/no question?

In sign language, non-manual markers that indicate a yes/no question often involve specific facial expressions. The correct non-manual marker for yes/no questions is:

• eyebrows up

So the answer is: eyebrows up.

The other options, such as "eyebrows down," "yes face," and "no face," do not typically indicate yes/no questions in sign language.