Final answer:
Simultaneity is the concept from special relativity associated with the clock synchronization problem, emphasizing that the concept of 'now' is relative to the observer's frame of reference.
Step-by-step explanation:
The concept from special relativity that is highlighted by the clock synchronization problem is D) Simultaneity. Special relativity, formulated by Albert Einstein, demonstrates that simultaneity is not an absolute concept but rather is relative to the observer's frame of reference.
The synchronization of clocks in different reference frames becomes an intricate task because the finite speed of light causes events that appear simultaneous in one frame to potentially appear as occurring at different times in another. This underlines the relativity of time, and demonstrates that what is 'now' for one observer might not be 'now' for another when they are moving relative to each other at significant fractions of the speed of light.