The most important reason an object is observed to be shorter in a frame where it is moving than in a frame where it is at rest is that
A) The force of motion strongly compresses an object that is moving at relativistic speeds.
B) "Simultaneity" is not a frame-independent concept.
C) The measuring sticks used by the moving observer are Lorentz-contracted.
D) The clocks used by the moving observer run slower.

Question

Quizplus · Accepted Answer

Lorentz contraction, which causes an object to appear shorter in a frame where it is moving, is fundamentally tied to the relativity of simultaneity. The concept that simultaneous events in one frame may not be simultaneous in another directly affects the measurement of lengths, leading to the observed contraction. 
This is the main point of section R6.3.
Section R6.3: What Causes the Contraction?
This contraction effect is entirely due to the fact that observers in different frames disagree about clock synchronization. Since different observers will see different pairs of events marking out the object's ends to be simultaneous, they cannot agree on the object's length.
There is an analogy for this in plane geometry. Consider a road with parallel sides. The road's east-west width depends on the orientation of one's coordinate system, even though the road's physical reality is unchanged. Similarly, an object's projection on a frame's spatial axis (which is what its length really is) is frame-dependent, even though its four-dimensional reality is unchanged.

The Most Important Reason an Object Is Observed to Be