The reason for double helical structure of DNA is the operation of:

The question asks for the reason behind the double helical structure of DNA. DNA, or deoxyribonucleic acid, has a characteristic double helix shape, resembling a twisted ladder. This structure is stabilized by specific interactions between the nitrogenous bases on opposite strands.

The backbone of DNA consists of sugar and phosphate groups, while the rungs of the ladder are formed by pairs of nitrogenous bases: adenine (A) with thymine (T), and guanine (G) with cytosine (C). The force that holds these base pairs together is hydrogen bonding. Specifically:

• Adenine and thymine form two hydrogen bonds: $$ \text{A} \cdots \text{H} - \text{N} \quad \text{and} \quad \text{N} - \text{H} \cdots \text{O} $$ (between N-H of adenine and O of thymine, and N of adenine and H-N of thymine).
• Guanine and cytosine form three hydrogen bonds: $$ \text{O} \cdots \text{H} - \text{N}, \quad \text{N} - \text{H} \cdots \text{O}, \quad \text{and} \quad \text{N} - \text{H} \cdots \text{N} $$ (involving carbonyl and amino groups).

Hydrogen bonding is an intermolecular force where a hydrogen atom bonded to an electronegative atom (like N or O) is attracted to another electronegative atom. This provides directionality and specificity, ensuring complementary base pairing (A with T, G with C), which is crucial for the double helix formation.

Now, evaluating the other options:

• Option A: Van der Waals forces - These are weak, non-specific attractions between all atoms/molecules. While they contribute to the overall stability of the DNA helix by acting between stacked bases, they are not the primary force for base pairing.
• Option B: Electrostatic attractions - These involve interactions between charged groups. In DNA, the phosphate backbone has negative charges, which are neutralized by ions (e.g., Mg²⁺) or proteins, but this does not hold the base pairs together.
• Option D: Dipole-Dipole interactions - These occur between polar molecules but are weaker and less specific than hydrogen bonding. Hydrogen bonding is a stronger subset of dipole-dipole interactions, but the term "dipole-dipole" alone does not capture the specificity of base pairing in DNA.

Thus, hydrogen bonding is the key interaction responsible for the complementary base pairing that maintains the double helical structure of DNA.

Hence, the correct answer is Option C.