**Core Concept**
The cell cycle is a complex process of cell division, consisting of interphase and the mitotic phase. During interphase, the cell grows, replicates its DNA, and prepares for cell division. Interphase is further divided into three phases: Gap 1 (G1), Synthesis (S), and Gap 2 (G2).

**Why the Correct Answer is Right**
The correct answer is the Synthesis (S) phase, where the cell's genetic material is replicated, and the DNA content is doubled. This occurs through the process of DNA replication, where the double helix is unwound, and each strand serves as a template for the synthesis of a new complementary strand by DNA polymerase. The S phase is a crucial step in the cell cycle, as it ensures that each daughter cell receives a complete set of chromosomes.

**Why Each Wrong Option is Incorrect**
* **Option A:** Gap 1 (G1) is the first phase of interphase, where the cell grows and prepares for DNA replication. However, it is not the phase where cellular content is doubled.
* **Option B:** Gap 2 (G2) is the final phase of interphase, where the cell prepares for cell division. While it is a critical phase, it is not the phase where the cell's DNA content is doubled.
* **Option C:** The Mitotic phase is the part of the cell cycle where the cell divides into two daughter cells. This phase is divided into prophase, metaphase, anaphase, and telophase, but it is not the phase where the cell's DNA content is doubled.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the cell cycle is tightly regulated, and any disruption can lead to uncontrolled cell growth, resulting in cancer. The S phase is a critical checkpoint in the cell cycle, and its dysregulation can lead to genomic instability and cancer.

**Correct Answer: C. Synthesis (S) phase. The Synthesis (S) phase is the phase of the cell cycle where cellular content is doubled.**