What term describes a flat surface from which an angle is measured?

The term "Reference Plane" is the appropriate choice because it refers specifically to a flat surface utilized as a baseline from which angles or measurements are determined in various fields, including health science and engineering. This concept is essential for establishing a standardized point of reference that ensures consistency in measurements and can significantly impact the accuracy of data collection, particularly in anatomical studies or medical imaging.

Other terms presented do not fulfill this specific requirement. For instance, a "Vertical Axis" typically pertains to an orientation or directional line that is perpendicular to the ground, not a flat surface. "Horizontal Line" represents a straight line that runs left to right, which is also not explicitly a surface used for measuring angles. "Measurement Surface" lacks the specificity and established definition that "Reference Plane" has in professional terminology. Hence, "Reference Plane" is the most precise term for describing a foundational flat surface from which angles are assessed.