Answer
(a) and (d) are the correct options.
Work Step by Step
The two observers are moving relative to one another, so they measure the same speed relative to one another. Similarly, according to the second postulate of the special theory of relativity, the speed of light is constant in a vacuum in all inertial frames of references. Thus, the two observers will always measure the same values -- that is, (a) their relative speed and (d) the speed of light in a vacuum.
