* Projectile motion describes the motion of an object that is launched into the air and then moves under the sole influence of gravity. This motion involves both horizontal and vertical components.

Let's break down the other options:

* a. horizontal velocity: This is a component of projectile motion. While the horizontal velocity remains constant (ignoring air resistance), it's not the entire motion.

* b. directrix: A directrix is a concept in geometry, specifically related to conic sections (like parabolas). While the path of a projectile can be described by a parabola, the directrix itself doesn't define the motion.

* c. vertical: This is also a component of projectile motion. The vertical motion is affected by gravity, causing the object to accelerate downwards.

In summary: Projectile motion is the specific type of motion that combines both horizontal and vertical components under the influence of gravity.