Answer
$(c)$ The speed of light in a vacuum
$(d)$ The relative speed between the observers
Work Step by Step
According to the special theory of relativity, the speed of light is same in any inertial frame of reference , so both will measure the same speed of light $(c)$.
Also , because both move relative to each other , their relative speed must be the same from both reference frames , only the opposite direction $(d)$.
