Concept:
Projectile motion is the motion of an object projected into the air under the influence of gravity. Once the object is launched, gravity continuously acts on it and causes the object to follow a curved path called a trajectory.
The acceleration due to gravity is responsible for changing the vertical component of motion.
Step 1: Understand the forces acting in projectile motion.
In projectile motion, air resistance is generally neglected and gravity is the primary force acting on the object.
Step 2: Identify the most important factor.
The curved path and downward motion of the projectile are caused by:
\[
\text{Gravity}
\]
Hence, the correct answer is:
\[
\boxed{(D)\ \text{Gravity}}
\]