**Core Concept**
The correct answer relates to the application of Polymerase Chain Reaction (PCR), a laboratory technique used for amplifying specific DNA sequences.

**Why the Correct Answer is Right**
PCR is a widely used molecular biology technique that enables the rapid amplification of specific DNA sequences. This is achieved through an exponential amplification process, where the target DNA sequence is denatured, annealed to primers, and extended by DNA polymerase. The process involves multiple cycles of denaturation, annealing, and extension, resulting in an exponential increase in the number of copies of the target DNA sequence. This technique is crucial for various applications, including diagnostic testing, forensic analysis, and research.

**Why Each Wrong Option is Incorrect**
**Option A:** PCR is not primarily used for DNA sequencing. While PCR can be used as a starting material for sequencing, it is not its primary application.

**Option B:** PCR is not used for RNA amplification. Although there are techniques like reverse transcription PCR (RT-PCR) that can be used to amplify RNA, PCR itself is specifically designed for DNA amplification.

**Option C:** PCR is not used for protein synthesis. While PCR can be used to amplify DNA sequences that encode proteins, the technique itself does not directly synthesize proteins.

**Clinical Pearl / High-Yield Fact**
PCR is a highly sensitive and specific technique, capable of detecting minute amounts of DNA. However, it is essential to note that PCR can also amplify contaminants, such as DNA from bacteria or other sources, which can lead to false-positive results.

**Correct Answer: D. Amplification of specific DNA sequences.**