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.