**Core Concept**
The patient's presentation of hydrops and scarring of the cornea indicates a severe and irreversible corneal disease, necessitating a full-thickness corneal replacement. This scenario is best addressed with a surgical intervention that replaces the entire thickness of the cornea.

**Why the Correct Answer is Right**
Penetrating Keratoplasty (PK) is a surgical procedure where the entire thickness of the cornea is replaced with a donor cornea. This is the most suitable treatment option for patients with advanced corneal disease, such as keratoconus with hydrops, and corneal scarring. PK allows for the removal of the diseased corneal tissue and its replacement with a healthy donor cornea, restoring vision and improving the patient's quality of life. The procedure involves a full-thickness incision in the cornea, allowing for the exchange of the diseased tissue with the donor cornea.

**Why Each Wrong Option is Incorrect**
**Option A:** Endothelial lamellar Keratoplasty (ELK) is a partial-thickness corneal transplant that replaces only the endothelial layer of the cornea. This procedure is typically used for diseases affecting the endothelium, such as Fuchs' endothelial dystrophy, and is not suitable for patients with full-thickness corneal scarring.

**Option B:** Deep anterior lamellar keratoplasty (DALK) is a partial-thickness corneal transplant that replaces the anterior layers of the cornea, preserving the endothelial layer. This procedure is used for diseases affecting the anterior cornea, such as keratoconus, but not for full-thickness corneal scarring.

**Option C:** Collagen cross-linking is a non-surgical procedure that strengthens the cornea by creating new covalent bonds between collagen fibers. While this treatment can be effective for early stages of keratoconus, it is not a suitable option for patients with advanced corneal disease and full-thickness scarring.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the choice of surgical procedure depends on the extent and location of the corneal disease. A thorough preoperative evaluation, including corneal imaging and mapping, is crucial to determine the most suitable treatment modality.

**✓ Correct Answer: A. Penetrating Keratoplasty**