C

Step 1: Analyzing the motion of the ball, it is important to note that while the ball is in the air, it is subjected to gravitational force acting downwards.
Step 2: The horizontal motion of the ball is uniform since there are no horizontal forces acting upon it (assuming air resistance is negligible). According to the principles of physics, this means that the horizontal component of velocity remains constant throughout the flight.
Step 3: On the other hand, the vertical component of velocity changes due to gravity: it increases downwards as the ball falls and decreases as it rises.
Step 4: The speed of the ball changes throughout its motion, being maximum at the lowest point and minimum at the highest point of its trajectory.
Step 5: Thus, the only quantity that remains constant during the entire motion is the horizontal component of the velocity.

Therefore, the correct answer is C.