**Core Concept**
The two strands of DNA are held together by a type of chemical bond that is essential for the stability and replication of genetic material. This bond is known as a hydrogen bond, which is formed between the bases of the two strands. The specific interaction between the bases is crucial in maintaining the double helix structure of DNA.

**Why the Correct Answer is Right**
The correct answer is **hydrogen bond**. Hydrogen bonds are weak electrostatic attractions between the positively charged hydrogen atoms of one base and the negatively charged oxygen or nitrogen atoms of another base. In DNA, adenine (A) forms hydrogen bonds with thymine (T), while guanine (G) forms hydrogen bonds with cytosine (C). These base pairing interactions are specific and complementary, allowing the two strands of DNA to be held together in a stable double helix structure. The hydrogen bonds are responsible for the stability of the DNA molecule and are crucial for its replication and transcription.

**Why Each Wrong Option is Incorrect**
**Option A:** **Van der Waals forces** are weak intermolecular forces that can contribute to the stability of DNA, but they are not responsible for holding the two strands together.

**Option B:** **Covalent bonds** are strong chemical bonds that form between atoms within a molecule, but they are not the type of bond that holds the two strands of DNA together.

**Option C:** **Disulfide bonds** are covalent bonds that form between sulfur atoms in certain proteins, but they are not relevant to the structure of DNA.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the base pairing interactions in DNA are specific and complementary, with adenine pairing with thymine and guanine pairing with cytosine. This specificity is crucial for the accurate replication and transcription of genetic material.

**Correct Answer:** C. Hydrogen bonds.