Final answer:
CDK6 must bind to and be activated by cyclin D in order to be active and regulate the cell cycle.
Step-by-step explanation:
To be active, CDK6 must bind to and be activated by cyclin D. The cyclin D-CDK6 complex is essential for regulating the progression of the cell cycle, particularly during the G1 phase. This complex is involved in phosphorylating target proteins and promoting the transition from G1 to S phase, initiating DNA synthesis.