In the Scaled Agile Framework (SAFe), Agile Teams work together as part of an Agile Release Train (ART). One of the defining features that unites all teams within an ART is synchronization to a common cadence. This means all teams follow the same iteration schedule---planning, executing, reviewing, and retrospecting in harmony.

The common cadence provides a steady rhythm that simplifies coordination and dependency management across multiple teams. It ensures alignment by creating predictable, synchronized events such as PI Planning, System Demos, and Inspect & Adapt workshops. This synchronization allows cross-team integration and transparency, enabling teams to deliver coherent system-level increments and support scaled agility.

From SAFe guidance:

''Teams on the Agile Release Train (ART) align to a common cadence and synchronize their iterations, which facilitates system-level integration and provides regular opportunities for assessment and adaptation.''

Therefore, the correct answer is D. Synchronization to a common cadence, as it represents the key organizing attribute shared by all teams on an ART.