Constant Quantity during upward motion of a ball:
During the upward motion of a ball thrown vertically, the only quantity that remains constant is the acceleration due to gravity.

Explanation:
When a ball is thrown vertically upwards, it experiences various changes in its motion. Let's analyze each of the given quantities to determine which one remains constant.

Acceleration due to gravity:
The acceleration due to gravity, denoted by 'g', is the rate at which an object falls towards the Earth under the influence of gravity. Near the surface of the Earth, this value is approximately 9.8 m/s². During the upward motion of a ball, the acceleration due to gravity acts in the opposite direction of the motion, slowing down the ball's upward velocity. However, the magnitude of the acceleration remains constant throughout the motion. Therefore, option 'A' is correct.

Displacement:
Displacement refers to the change in position of an object. During the upward motion of a ball, its displacement continuously changes as it moves away from the point of release. Initially, the displacement is positive, indicating an upward movement. However, as the ball reaches its highest point and starts falling back downwards, the displacement becomes negative, indicating a downward movement. Therefore, the displacement is not constant during the entire motion. Hence, option 'B' is incorrect.

Velocity:
Velocity is a vector quantity that refers to the rate of change of displacement with respect to time. During the upward motion of a ball, its velocity continuously changes. Initially, the velocity is positive, indicating an upward movement. However, as the ball reaches its highest point and starts falling back downwards, the velocity becomes negative, indicating a downward movement. Therefore, the velocity is not constant during the entire motion. Hence, option 'C' is incorrect.

Therefore, the only quantity that remains constant during the upward motion of a ball is the acceleration due to gravity, making option 'A' the correct answer.