Final answer:
Special relativity differs from Galilean relativity primarily in its second postulate, which states that the speed of light is constant in all inertial frames, contrary to the additive velocities of Galilean relativity.
Step-by-step explanation:
Special relativity differs significantly from Galilean relativity in its treatment of space, time, and velocity. Galilean relativity assumes that the laws of motion are the same in all inertial frames and that velocities add linearly; this is reflected in the Galilean transformation, which relates the coordinates and time between two frames moving at a constant velocity relative to each other. However, it does not consider the constancy of the speed of light.
The first postulate of special relativity, much like Galilean relativity, states that the laws of physics are invariant in all inertial frames of reference. Nevertheless, Einstein's second postulate introduces a vital difference: it asserts that the speed of light is the same for all observers, regardless of their motion or the motion of the light source, which directly contradicts Galilean ideas about relative velocities.
The correct answer to the question is b. The second postulate: the speed of light is the same in all inertial reference frames, this is a postulate that departs from Galilean relativity's assumptions about relative motion.