**Core Concept**
The World Health Organization (WHO) classification of vitamin A deficiency is based on the clinical manifestations, particularly in the eye. Vitamin A deficiency can lead to night blindness, corneal ulcers, and eventually, blindness. The classification system helps in assessing the severity of vitamin A deficiency.

**Why the Correct Answer is Right**
The correct answer corresponds to the stage where there is a significant involvement of the cornea, indicating a severe deficiency. Corneal ulcers involving half of the corneal surface suggest a profound impact of vitamin A deficiency on epithelial integrity and healing. Vitamin A is crucial for maintaining the health of the cornea, and its deficiency can lead to severe ocular complications.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not correspond to the specific stage of vitamin A deficiency characterized by corneal ulcers involving half of the corneal surface.
**Option B:** Similarly, this option does not match the description provided.
**Option D:** This option is incorrect as it does not align with the stage of severe vitamin A deficiency with significant corneal involvement.

**Clinical Pearl / High-Yield Fact**
Vitamin A deficiency is a significant public health problem in many parts of the world, particularly affecting children. Early recognition and treatment of vitamin A deficiency can prevent severe and irreversible ocular complications.

**Correct Answer:** C.