answer:answer is second postulate (invariance of c)
explanation:postulate (principle of relativity) laws of physics take the same form in all inertial frames of reference. postulate (invariance of c) measured in any inertial frame of reference, light is always propagated in empty space with a definite velocity c that is independent of the state of motion of the emitting body. or the speed of light in free space has the same value c in all inertial frames of reference.
the two-postulate basis for special relativity is the one historically used by einstein, and it remains the starting point today.