Answer: Within any frame of reference that is accelerating
Special relativity was proposed on 1905 by Einstein, who developed his theory based on the following two postulates:
1. The laws of physics are the same in all inertial systems. There is no preferential system.
2. The speed of light in vacuum has the same value for all inertial systems.
Focusing on the first postulate, it can be affirmed that any measurement on a body is made with reference to the system in which it is being measured.
Now, taking into account that an inertial reference system is the one that complies with the principle of inertia:
"For a body to have acceleration, an external force must act on it."
The correct answer is
Within any frame of reference that is accelerating
