**Conservation of Momentum in Collisions**

Momentum is defined as the product of an object's mass and its velocity. It is a vector quantity, meaning it has both magnitude and direction. In a collision, momentum is a crucial physical quantity that is conserved.

**Elastic Collision**
In an elastic collision, both kinetic energy and momentum are conserved. When two objects collide elastically, they bounce off each other without any loss of kinetic energy. The total momentum before the collision is equal to the total momentum after the collision.

**Inelastic Collision**
In an inelastic collision, only momentum is conserved, while kinetic energy is not. In this type of collision, the objects stick together or deform upon collision, resulting in a loss of kinetic energy. Even though the objects may lose or gain kinetic energy, the total momentum of the system remains constant.

**Explanation of the Correct Answer (Option C)**
The correct answer is option C, momentum. This is because momentum is always conserved in both elastic and inelastic collisions. Whether the collision is elastic or inelastic, the total momentum of the system before the collision is equal to the total momentum after the collision.

**Why is Momentum Conserved?**
The conservation of momentum can be understood by considering Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. When two objects collide, the force exerted by one object on the other causes a change in momentum. However, according to Newton's third law, the force exerted by the second object on the first is equal in magnitude and opposite in direction. As a result, the total momentum of the system remains constant.

**Impulse and Momentum**
Impulse is the change in momentum of an object and is equal to the force applied multiplied by the time over which it acts. In a collision, the impulse experienced by each object is equal and opposite, resulting in a change in their momentum. The total impulse is zero, and therefore, the total momentum of the system is conserved.

**Conclusion**
In summary, momentum is a physical quantity that is conserved in both elastic and inelastic collisions. While kinetic energy is conserved in elastic collisions, it is not conserved in inelastic collisions. The conservation of momentum can be explained by Newton's third law of motion, which states that the forces exerted on each other by the colliding objects are equal in magnitude and opposite in direction.