**Core Concept**
The question pertains to the structural and thermodynamic stability of DNA, which is a fundamental concept in biochemistry. Thermo-stability in DNA refers to its ability to withstand high temperatures without undergoing denaturation. This property is crucial for the replication and transcription processes.

**Why the Correct Answer is Right**
The correct answer, B. GEC, refers to the guanine-cytosine (G-C) base pairs. These base pairs contribute significantly to the thermodynamic stability of DNA due to the following reasons:
- Guanine and cytosine form three hydrogen bonds between them, which is more than the two hydrogen bonds formed between adenine and thymine (A-T).
- The presence of a six-membered ring in guanine and a five-membered ring in cytosine allows for a more stable stacking interaction between the bases, which further enhances the thermodynamic stability of DNA.
- The G-C base pairs are also more resistant to thermal fluctuations, which helps maintain the double helix structure of DNA.

**Why Each Wrong Option is Incorrect**
**Option A:** A-T base pairs contribute to the structural stability of DNA but not to its thermodynamic stability. The two hydrogen bonds between adenine and thymine are weaker compared to the three hydrogen bonds between guanine and cytosine.

**Option C:** The term "molecular base" is ambiguous and does not specifically refer to any particular base pair. While all base pairs contribute to the structural stability of DNA, the question specifically asks about thermodynamic stability.

**Option D:** The parallel arrangement of DNA strands contributes to its structural stability but not to its thermodynamic stability. The double helix structure of DNA is maintained by the hydrogen bonding between base pairs, not by the parallel arrangement of the strands.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the G-C content of DNA affects its melting temperature (Tm). DNA with a higher G-C content has a higher Tm, indicating greater thermodynamic stability.

**✓ Correct Answer: B. GEC**