Special relativity says the speed of light is the same for every observer, whatever their own motion. Why does that single postulate force time and length to be observer-dependent?

D. Because speed is distance over time, so those must be what varies.

The argument is almost arithmetic: speed is distance over time, and if two observers in relative motion both measure c, their distances and times cannot both agree. Nothing is happening to the clocks, a moving clock is in perfect order in its own frame. And the effects are routine: atmospheric muons reach the ground only because their lifetimes are dilated.