**Core Concept**
Keratin is a type of protein that forms the main structural component of hair, nails, and the outer layer of skin. The strength of keratin in nails is attributed to the specific type of chemical bonds that hold the protein molecules together.

**Why the Correct Answer is Right**
Keratin in nails is exceptionally strong due to the presence of disulfide bonds. These covalent bonds are formed between the sulfur atoms of two cysteine amino acids, resulting in a strong and stable structure. Disulfide bonds are crucial for maintaining the integrity and rigidity of keratin fibers, making nails resistant to wear and tear. The arrangement of disulfide bonds in keratin provides a high degree of cross-linking, which contributes to the exceptional strength of nail keratin.

**Why Each Wrong Option is Incorrect**
**Option B:** Van der Waals forces are weak intermolecular forces that contribute to the overall structure of proteins, but they are not responsible for the exceptional strength of nail keratin. Van der Waals forces are relatively weak and are not sufficient to explain the remarkable strength of keratin fibers.
**Option C:** Ionic bonds involve the electrostatic attraction between oppositely charged ions. While ionic bonds can contribute to protein structure, they are not the primary reason for the strength of keratin in nails. Disulfide bonds are a more significant factor in maintaining the structural integrity of keratin.
**Option D:** Calcification refers to the deposition of calcium salts in tissues, which is not relevant to the strength of keratin in nails. Keratin is a protein, not a calcium-containing substance, and its strength is not due to calcification.

**Clinical Pearl / High-Yield Fact**
Remember that the specific arrangement of disulfide bonds in keratin is responsible for its exceptional strength and rigidity. This is a key factor in the development of nail disorders, such as nail brittleness or fragility, which can result from disruptions in disulfide bond formation or stability.

**✓ Correct Answer: A. Disulphide**