C

Step 1: Understand the concept of centrifugal force. Centrifugal force is often described as an apparent force that acts outward on a mass moving in a circular path and is directed away from the center of rotation.
Step 2: Identify the perspective from which centrifugal force is observed.
An observer who is moving with the particle (reference frame of the particle) will perceive the outward force acting on the particle due to its inertia.
Step 3: Contrast this with other observers:
- An observer at the center of circular motion will not experience this force as they are in a non-inertial reference frame.
- An outside observer will not feel this force since they can see the circular motion from an inertial frame, where only centripetal force acts.
Therefore, the centrifugal force is an inertial force from the perspective of an observer who is moving with the particle experiencing it.
Conclusion: The correct choice is an observer who is moving with the particle which is experiencing the force (Option C).