**Core Concept**
RNA primers are short RNA sequences required for DNA replication, particularly in the initiation of DNA synthesis. They serve as a starting point for DNA polymerase to add nucleotides to, allowing for the extension of the new DNA strand. The synthesis of RNA primers is a crucial step in the replication process.

**Why the Correct Answer is Right**
RNA primers are synthesized by an enzyme called primase, which is a type of RNA polymerase. Primase recognizes specific sequences on the template DNA, such as the origin of replication, and adds short RNA primers to initiate DNA synthesis. This process is essential for the replication of DNA, as DNA polymerase can only add nucleotides to an existing strand.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because primase is the specific enzyme responsible for synthesizing RNA primers, not DNA polymerase.
**Option B:** Incorrect because helicase is an enzyme involved in unwinding the DNA double helix, not in synthesizing RNA primers.
**Option C:** Incorrect because ligase is an enzyme that seals gaps between Okazaki fragments, not involved in RNA primer synthesis.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that RNA primers are synthesized by primase, and their presence is necessary for DNA polymerase to initiate DNA synthesis. This understanding is crucial in understanding the process of DNA replication and its regulation.

**Correct Answer: D. Primase**