**Core Concept**
Keratoconus is a progressive eye disease characterized by thinning of the cornea, leading to conical protrusion and vision impairment. The pathophysiology involves a combination of genetic predisposition, mechanical stress, and enzymatic degradation of the corneal tissue.

**Why the Correct Answer is Right**
Rigid gas permeable (RGP) contact lenses are used to prevent keratoconus by applying gentle pressure to the cornea, thereby reducing mechanical stress and promoting corneal stability. The pressure exerted by RGP lenses stimulates the production of collagen, a key structural protein in the cornea, which helps to strengthen the tissue and prevent further thinning. By controlling the shape of the cornea, RGP lenses also reduce the risk of conical protrusion and vision impairment associated with keratoconus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because soft contact lenses, which are more flexible and conformable than RGP lenses, may actually exacerbate keratoconus by allowing the cornea to protrude further and causing increased mechanical stress.
**Option B:** This option is incorrect because corneal cross-linking, a treatment used to stabilize the cornea in advanced keratoconus, is not a preventive measure.
**Option C:** This option is incorrect because vitamin supplements, while essential for overall eye health, do not specifically prevent keratoconus.

**Clinical Pearl / High-Yield Fact**
It is essential to note that keratoconus can be asymptomatic in its early stages, and regular eye exams are crucial for early detection and prevention.

**Correct Answer: D. Rigid gas permeable (RGP) contact lenses.**