The correct answer is B) coaltitude of the body. The radius of a circle of equal altitude of a celestial body is equal to the coaltitude of that body. The coaltitude is the complement of the altitude, meaning it is the angle between the celestial body and the zenith (the point directly overhead). This relationship is derived from the geometry of the celestial sphere and is a fundamental principle in celestial navigation using the altitude of celestial bodies. The other options are incorrect because they do not directly relate to the radius of the circle of equal altitude. The polar distance, codeclination, and altitude are different celestial navigation concepts that are not directly linked to the radius of the circle of equal altitude.