**Core Concept**
Neovascularisation is a process by which new blood vessels form in response to various stimuli, including ischemia, inflammation, and hypoxia. In the context of ophthalmology, neovascularisation can occur in response to retinal ischemia, leading to the formation of new, fragile blood vessels that can cause vision loss.

**Why the Correct Answer is Right**
Central vein obstruction, also known as central retinal vein occlusion (CRVO), is a condition characterized by the blockage of the central retinal vein. This blockage leads to retinal ischemia, which triggers the release of various growth factors, including vascular endothelial growth factor (VEGF). These growth factors stimulate the formation of new blood vessels, leading to neovascularisation. The new blood vessels can cause further vision loss by leaking fluid and causing retinal detachment.

**Why Each Wrong Option is Incorrect**
**Option B:** Central retinal artery obstruction is a condition characterized by the blockage of the central retinal artery, which leads to retinal ischemia. However, the primary response to retinal ischemia in central retinal artery obstruction is not neovascularisation, but rather retinal infarction and atrophy.

**Option C:** Branch retinal vein obstruction is a condition characterized by the blockage of a branch retinal vein. While branch retinal vein obstruction can cause retinal ischemia, it is less likely to trigger neovascularisation compared to central vein obstruction.

**Option D:** All of the above is incorrect because central vein obstruction is the most likely condition to trigger neovascularisation among the options listed.

**Clinical Pearl / High-Yield Fact**
Neovascularisation in central vein obstruction is often associated with the development of vitreous hemorrhage, which can lead to sudden vision loss. Therefore, prompt recognition and treatment of central vein obstruction are crucial to prevent vision loss.

**✓ Correct Answer: A. Central vein obstruction**