**Core Concept**
The melting temperature of DNA is a critical parameter in molecular biology that refers to the temperature at which double-stranded DNA denatures into single strands. This concept is closely related to the base pairing between nucleotides and the stability of the DNA double helix.

**Why the Correct Answer is Right**
The melting temperature of DNA is directly proportional to the GC (guanine-cytosine) pairing, as these base pairs form three hydrogen bonds, which are stronger than the two hydrogen bonds between AT (adenine-thymine) pairs. As a result, GC-rich regions of DNA are more stable and require higher temperatures to denature. This principle is crucial in understanding DNA hybridization, PCR, and other molecular biology techniques. The stability of GC pairs is due to the higher enthalpy change (∆H) and melting temperature (∆Tm) associated with these interactions.

**Why Each Wrong Option is Incorrect**
**Option B:** AT pairing is incorrect because these base pairs have weaker hydrogen bonds (two hydrogen bonds) compared to GC pairs, resulting in lower melting temperatures. AT-rich regions of DNA are less stable and require lower temperatures to denature.
**Option C:** The length of DNA is incorrect because the melting temperature of DNA is not directly proportional to its length. While longer DNA molecules may have a higher melting temperature due to increased GC content, this is not a direct correlation.

**Clinical Pearl / High-Yield Fact**
When working with DNA, it's essential to consider the GC content of the target sequence to optimize melting temperatures for hybridization or PCR reactions. A higher GC content typically requires higher annealing temperatures to ensure specificity and efficiency.

✓ Correct Answer: A. GC Pairing.