How does a projectile move in terms of force acting on it?

A projectile moves primarily under the influence of gravity once it has been launched. When an object is projected into the air, it typically follows a curved trajectory known as a parabolic path. The only force acting on it during its flight, after being launched, is the force of gravity pulling it downward.

Initially, when the projectile is launched, it may have an initial force (thrust) that propels it. However, once the projectile is in motion and the influence of that initial force has diminished, gravity becomes the sole force acting on it until it reaches the ground. This is why the correct answer focuses on gravity; it clearly delineates the behavior of the projectile once it's in its flight path, unaffected by continuous propelling forces or external engines.

Additionally, options that suggest continuous force or accelerating force imply persistent external influence which does not apply to an object in free fall after its initial launch. The concept that a projectile moves under gravity only accurately reflects the fundamental physics principles governing projectile motion in an idealized scenario where air resistance is negligible.