What instrument is used to measure the angle of a joint's range of motion?

The goniometer is the correct choice because it is specifically designed to measure the angle of a joint's range of motion. This tool provides clinicians and therapists with precise measurements of joint angles, which are essential for assessing flexibility, tracking progress in rehabilitation, and determining the extent of joint impairment. Goniometers can be either manual, where a physical protractor-like device is used, or digital, offering more advanced measurement capabilities.

A tensiometer is used to measure tension or force, typically in muscles or other structures, rather than angles. Protractors are general measuring tools used in various fields, primarily in geometry, to measure angles in two dimensions, but not specifically for joint motion. A flexometer, while it relates to measuring flexibility, does not specifically refer to the angular measurement of joints and is less commonly used compared to a goniometer. Thus, the goniometer stands out as the most appropriate instrument for evaluating the angle of a joint's range of motion.