**Core Concept**
DNA replication is the process by which a cell makes an exact copy of its DNA before cell division. It involves the unwinding of the double helix structure, synthesis of new complementary strands, and reassembly of the double helix. The process is crucial for the transmission of genetic information from one generation of cells to the next.

**Why the Correct Answer is Right**
DNA replication occurs in two main phases: initiation and elongation. The initiation phase involves the unwinding of the double helix at specific regions called origins of replication, where an enzyme called helicase unwinds the DNA, and another enzyme called primase adds RNA primers to the template strands. The elongation phase involves the synthesis of new complementary strands by an enzyme called DNA polymerase, which reads the template strands and matches the incoming nucleotides to the base pairing rules (A-T and G-C). The process continues until the replication fork is complete, and the new double helix is formed.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because DNA replication is a complex process that cannot be summarized in a single phase. The two-phase model provides a more accurate representation of the process.
**Option B:** This option is incorrect because the process of DNA replication involves many enzymes and molecular machinery, but it is not solely a single phase. The initiation and elongation phases work together to complete the replication process.
**Option C:** This option is incorrect because the process of DNA replication involves the synthesis of new complementary strands, which requires the unwinding of the double helix and the addition of nucleotides. A single phase would not be sufficient to describe this complex process.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that DNA replication is a highly regulated process that involves many enzymes and molecular machinery. Any errors in the replication process can lead to mutations and genetic disorders.

**Correct Answer: D. Two phases.