Answer: Option A, speed of light.
Explanation: As the object reaches the speed of light, the length of the object will contract, this is called the Lorentz contraction. Also, the object will experience the time slower than if it were at rest, so an observer and the object will experience the time in a different way, and because of this the "rate of time passage" changes, so we already discarded option B, C, and D.
Now, the speed of the light is invariant, it is the same in any system and is a "physical constant", so it would not change, then the correct option is A.
