"relativistic speed" means the speed that is very high relative to the speed of light, therefore the correct answer is option C
the speed at which light travel is known as the speed of light, the speed of light in a vacuum has a value of 3×10⁸ m/s.
According to the relativity theory proposed by einstein, the speed which is near about the speed of light is known as the "relativistic speed"
Thus , "relativistic speed" means the speed that is very high relative to the speed of light, therefore the correct answer is option C
Learn more about the speed of light here
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