**Core Concept**
Real-time Reverse Transcriptase PCR (RT-PCR) is a sensitive and quantitative method for measuring gene expression by amplifying and detecting specific mRNA transcripts. This technique allows for the accurate quantification of gene expression levels by monitoring the amplification process in real-time.

**Why the Correct Answer is Right**
Real-time RT-PCR involves the reverse transcription of mRNA into cDNA, followed by amplification using PCR. The amplification process is monitored in real-time using fluorescent probes or dyes, allowing for the quantification of gene expression levels. This technique is highly sensitive and specific, making it an ideal method for accurate quantification of gene expression. The use of fluorescent probes or dyes enables the detection of the amplification products in real-time, eliminating the need for post-PCR analysis.

**Why Each Wrong Option is Incorrect**
**Option A:** Northern blot is a technique used for detecting specific RNA molecules, but it is not a quantitative method and does not allow for real-time monitoring of gene expression.

**Option B:** PCR (Polymerase Chain Reaction) is a technique used for amplifying DNA sequences, but it does not allow for the quantification of gene expression levels without additional modifications.

**Option D:** Reverse Transcriptase PCR (RT-PCR) is a technique used for amplifying cDNA from mRNA, but it does not involve real-time monitoring of the amplification process, making it less accurate for quantification of gene expression levels.

**Clinical Pearl / High-Yield Fact**
Real-time RT-PCR is a highly sensitive and specific technique for quantifying gene expression levels, making it an essential tool in molecular biology and gene expression studies.

**Correct Answer:**
✓ Correct Answer: C. Real-Time Reverse Transcriptase PCR