When an object is moving with uniform circular motion, the object’s tangential speed a. is circular. b. is perpendicular to the plane of motion. c. is constant. d. is directed toward the center of motion.

When an object is moving with uniform circular motion, the object's tangential speed is constant.

Option C

Explanation:

The angular velocity exhibited by an objects move in circular motion will have same magnitude but their direction will be different. At each point on the circular path the velocity's direction  tend to be tangential to the circle's circumference.This is why the angular velocity is also termed as tangential velocity or speed.

The angular velocity's magnitude is generally termed as tangential speed, a scalar quantity. So whenever an object is moving on a circular track or path the tangential speed's direction acting on the object will be in the tangential direction to the circular path in other words the speed will be acting in perpendicular direction to the motion plane.