**Core Concept:** The cell cycle is a series of events that lead to the division of a cell, including two main phases: the G1 phase (Gap 1) and the S phase (Synthesis). The cell cycle is regulated by various proteins and enzymes, with cyclin-dependent kinases (CDKs) playing a crucial role in controlling the transition between the phases. CDKs are activated by cyclins, which are proteins synthesized at specific times during the cell cycle.

**Why the Correct Answer is Right:** CDK4 and CDK6 are part of a group of CDKs known as G1-specific cyclin-dependent kinases. They are activated by cyclin D, which is synthesized during the G1 phase. Once activated, CDK4/6-cyclin D complex phosphorylates and inactivates the retinoblastoma protein (Rb), leading to the release of E2F transcription factors and progression to the S phase. The correct answer is **D** (CDK4/6-cyclin D complex).

**Why Each Wrong Option is Incorrect:**

A. CDK2-cyclin E complex is involved in the progression from G1 to S phase, but it is not specifically associated with the G2M transition.
B. Cyclin A and cyclin B are involved in the S phase and G2 phase, respectively, not G2M transition.
C. Cyclin D is involved in the G1 phase, not the G2M transition.

**Clinical Pearl:** Understanding the cell cycle and its regulation is crucial for understanding cancer development and therapeutic targets. Properly inhibiting these complexes during cancer treatment can selectively target rapidly dividing cells, while sparing normal cells with lower rates of division.