Question

A person says that he measured the acceleration of a particle to be non-zero even though no force was acting on the particle.
(a) He is a liar.
(b) His clock might have run slow.
(c) His metre scale might have been longer than the standard.
(d) He might have used a non-inertial frame.

Answer 1

(d) He might have used a non-inertial frame

If no force is acting on a particle and yet, its acceleration is non-zero, it means the observer is in a non-inertial frame.