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Speed is the distance traveled by a body in unit time. The speed of a body is independent of the direction in which it moves. The velocity of a body is the displacement in unit time. The velocity of a body is dependent on the direction of its motion. For instance, if a body goes up 5 m in a second or goes down 5 m in a second its speed is 5 m/s in both the cases, the velocity of the body on the other hand is has opposite signs.
When a body is in uniform circular motion, the distance being traveled by the body in unit time remains the same. The direction of motion is what changes continuously which means the velocity of the body is not the same.
The reason for a change in velocity is the centripetal acceleration of the body which results in a constant change in velocity and makes the body move in a circular path.
It is better to say "In a uniform circular motion the velocity continuously changes", because speed usually refers to the magnitude of the velocity and doesn't change in a uniform circular motion.
For this question, the answer is the direction of body's movement. The direction of the velocity continuously changes, while the magnitude of the velocity remains unchanged.
The question, as it stands, is flawed. Uniform circular motion, by definition, is motion in a circle at a uniform (constant) speed. It is thus incorrect to say that the speed continuously changes in uniform circular motion. To fix the question, the word “velocity” should be substituted in place of the word “speed.” Unlike speed, velocity is a vector, meaning it has both magnitude and direction. While the magnitude of the velocity (the speed) does not change, its direction does change. At any given instant during one of its revolutions, the body is moving in a different direction, thus its velocity is technically changing. This is known as centripetal acceleration.
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