The correct answer is C. All reference angles must be less than 90 degrees.
This is helpful when learning more complex topics including circular trigonometry and the unit circle, because the reference angle (typically denoted as theta prime) has the same trig values (sine and cosine) no matter which quadrant the angle is located in. Additionally, the reference angle is always the smallest angle possible between the terminal side of the angle and the x-axis; therefore, it must always be less than 90 degrees or else another smaller (and thus correct) reference angle could be made in another quadrant.
