Time dilation means that
a. time is unchanging regardless of the frame of reference.
b. the slowing of time in a moving frame of reference is only an illusion resulting from motion.
c. time really does pass more slowly in a frame of reference moving relative to a frame of reference at relative rest.
d. time really does pass more slowly in a rest frame of reference relative to a frame of reference that is moving.
e. no two clocks at rest can ever read the same time.