The correct answer is:

$$\textbf{Option A: Frictional force}$$

Here's why:

Conservative vs. Non-Conservative Forces:

Conservative Forces: These are forces for which the work done is independent of the path taken; they only depend on the initial and final positions. For conservative forces, you can define a potential energy function. Examples include Coulomb’s force, gravitational force, and the restoring force of a spring.

Non-Conservative Forces: These forces depend on the path taken and usually result in energy dissipation (often as heat). Friction is a typical example, and hence, it cannot be described solely by a potential energy function.

Example Explanations:

Coulomb’s Force: It is a conservative force because the work done in moving a charge in an electric field only depends on the difference in electric potential between two points.

Gravitational Force: It is also conservative. The potential energy associated with gravitational force is given by gravitational potential energy, for example, $$U = mgh$$ near the Earth’s surface.

Restoring Force: For a spring, the force is given by Hooke’s law: $$F = -kx$$ and the corresponding potential energy is $$U = \frac{1}{2}kx^2.$$ This is a conservative force.

Since friction is a non-conservative force that dissipates energy, it cannot be expressed in terms of potential energy.

Therefore, the correct answer is Option A: Frictional force.