**Core Concept**
Retinopathy of Prematurity (ROP) is a disease of the eye affecting prematurely born babies generally having birth weight less than 1250 grams. It's characterised by abnormal growth of blood vessels in the retina. ROP can be classified into 5 stages, with stages 1-3 being non-advanced and stages 4-5 being advanced.

**Why the Correct Answer is Right**
ROP stage 5 is characterised by a ridge with retinal fibrovascular proliferation and neovascularisation. This stage is the most severe form of ROP and can lead to retinal detachment and blindness. The retinal fibrovascular proliferation occurs due to the overgrowth of new, abnormal blood vessels that can invade the vitreous gel. This results in a ridge of fibrovascular tissue that can lead to retinal detachment.

**Why Each Wrong Option is Incorrect**
**Option A:** ROP stage 1 is characterised by a demarcation line between the normal and abnormal retina, but it does not involve retinal fibrovascular proliferation.

**Option B:** ROP stage 3 is characterised by the formation of extraretinal fibrovascular proliferation, but it is not the stage that involves a ridge with retinal fibrovascular proliferation.

**Option D:** ROP stage 4 is characterised by partial retinal detachment, but it does not involve a ridge with retinal fibrovascular proliferation.

**Clinical Pearl / High-Yield Fact**
ROP is a leading cause of childhood blindness in developed countries and can be prevented with early detection and treatment. Early screening is crucial in ROP management.

**Correct Answer:** C.