Which of the following factors affects the trajectory of a projectile?

The trajectory of a projectile is primarily influenced by gravity, which is the force that pulls the projectile towards the Earth. This pull affects the shape and path of the projectile's flight, causing it to follow a curved trajectory rather than a straight line. As the projectile travels upward, gravity slows it down, and eventually, the projectile begins to descend back to Earth, creating an arc in its path.

While wind direction can also influence the trajectory by altering the projectile's path during its flight, it does not determine the basic shape of the trajectory in the way that gravity does. Other factors such as barrel length and the color of the projectile do not significantly impact the trajectory. Barrel length may affect the velocity of the projectile when fired but does not directly alter its path once it is in motion. Similarly, the color of the projectile is irrelevant to its physical flight characteristics. Thus, gravity stands out as the primary factor influencing a projectile's trajectory.