The correct answer is: C. Force
Force is a push or pull that acts upon an object. It can cause the object to change its motion, or to deform. Force is always exerted by one object on another.
Inertia is the tendency of an object to resist any change in its motion. An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
Momentum is the product of an object’s mass and velocity. It is a vector quantity, meaning it has both magnitude and direction. Momentum is conserved, meaning that the total momentum of a system remains constant unless an external force acts on the system.
Velocity is the rate of change of an object’s position with respect to time. It is a vector quantity, meaning it has both magnitude and direction. Velocity is always in the same direction as the object’s motion.
Of the above, only force can never occur singly in nature. This is because force is always exerted by one object on another. For example, if you push on a wall, the wall pushes back on you with an equal and opposite force.