To start with, if you actually had no idea what the answer to this question was, you wouldn't pick B or C because x has to be close to 91°. In this particular quadrilateral, it's especially important that we focus on the fact, "x - 15." This means that x has to be higher initially to be subracted to equal 91, or the opposite angle.
In order to subtract to 91, the answer would have to be 106, or D. This is a hunch, but I have a suspicion that angle B and D are going to be the same, being that their on opposite sides of the figure.
Hope this helps! If you have further questions, let me know.
