**Core Concept**
The cell cycle is a complex process of cell division, consisting of four phases: G1 (Gap 1), S (synthesis), G2 (Gap 2), and M (mitosis). The sensitivity of cells to radiation varies across these phases, with some phases being more susceptible to damage than others.

**Why the Correct Answer is Right**
The most radiosensitive phase of the cell cycle is the G2 phase. This is because cells in G2 are preparing to enter mitosis, and their DNA is in a state of high metabolic activity, making them more vulnerable to radiation-induced damage. Additionally, the DNA replication machinery in G2 is more active, leading to increased DNA synthesis and repair, which can be disrupted by radiation, making cells more sensitive to its effects.

**Why Each Wrong Option is Incorrect**
* **Option A:** G1 phase is relatively radioresistant because cells are not actively replicating their DNA, and the DNA repair mechanisms are more efficient.
* **Option B:** S phase is also relatively radioresistant, as cells are actively replicating their DNA, but the DNA replication machinery is more efficient in this phase, allowing for better repair of radiation-induced damage.
* **Option D:** M phase is not the most radiosensitive phase, as cells are not actively replicating their DNA, and the DNA repair mechanisms are more efficient in this phase.

**Clinical Pearl / High-Yield Fact**
Radiation therapy is often used to target rapidly dividing cancer cells, which are more sensitive to radiation due to their higher metabolic activity and DNA replication rates.

**Correct Answer: C. G2 phase**