For a projectile launched horizontally, which of the following best describes the vertical component of a projectile's velocity? assume air resistance is negligible.

Assuming that air resistance is negligible in the given projectile motion, the x-component of the projectile motion should be constant. Whereas its vertical component should follow that of the free-fall because initially it is zero (due to the statement, launched horizontally). With that, all equations applicable to free falling objects should also be applicable to this type of problems. 

The masses of the objects and their densities