Introduction:
In physics, conservative forces are those that do not dissipate energy and do not depend on the path taken by an object. Non-conservative forces, on the other hand, are forces that dissipate energy and depend on the path taken. In this context, we need to determine which of the given forces is non-conservative in nature.

Frictional Force:
Frictional force is the force that opposes the motion of an object when it is in contact with a surface. This force is non-conservative in nature because it dissipates energy in the form of heat. When an object moves against the frictional force, work is done against it, and this work is converted into heat. The amount of work done against friction depends on the path taken by the object.

Explanation:
- The Coulomb force is the force of interaction between charged particles. It is a conservative force because it can be derived from a potential energy function.
- The gravitational force is the force of attraction between two objects with mass. It is also a conservative force because it can be derived from a potential energy function.
- The restoring force of an elastic spring is the force exerted by the spring when it is stretched or compressed. This force is conservative in nature because it can be derived from a potential energy function.

Conclusion:
Out of the given forces, the frictional force is the only force that is non-conservative in nature. It dissipates energy in the form of heat and depends on the path taken by an object. The other forces mentioned (Coulomb, gravitational, and restoring force of an elastic spring) are all conservative forces.