Which of the following is true regarding Projectile motion?
A) The horizontal velocity of the projectile is constant
B) The vertical velocity of the projectile is constant
C) Acceleration is not constant
D) Momentum is constant

Hint: Projectile motion occurs due to gravitation force and initial force given to the body. The vertical motion of the object is due to the gravitational force on the object. The fact that objects remain in motion is the factor important for giving the horizontal motion to the object.

Complete step by step answer:
The path followed by a particle in projectile motion as shown in the below figure.

Coming to the given options,
A) Horizontal velocity of the projectile is constant
As no gravitation force is acting on the horizontal component of the velocity that results in the horizontal velocity of the projectile constant. Whereas the vertical velocity component experiences an acceleration due to gravity.
B) Vertical velocity of the projectile is constant
The vertical velocity of the projectile is constant is not a true fact as the vertical component of velocity experiences acceleration due to gravity and hence it cannot be constant.
C) Acceleration is not constant
The vertical velocity of the projectile is not constant due to the acceleration that acts on the object is the acceleration due to gravity i.e. $9.81\;{\rm{m/}}{{\rm{s}}^2}$. So, this option is also false.
D) Momentum is constant
As it is known that the velocity of the object in the vertical direction is not constant and momentum depends on the velocity. So, it cannot be constant.

Therefore, Option (A) is correct.

Note:
The trajectory of a free-fall object under the effect of gravity is projectile. When a ball is thrown with an initial velocity follow the projectile motion under the effect of gravity once the ball is released.