**Core Concept**
The question is testing the understanding of **orbital compartment syndrome**, a condition where increased pressure within the orbit leads to compromised blood flow and potentially severe vision loss. This condition can arise from trauma, leading to swelling or hemorrhage within the confined space of the orbit.

**Why the Correct Answer is Right**
Given the late presentation of **hyperaemic sclera** and **unilateral proptosis** following a traumatic eye injury, the most likely diagnosis is related to increased orbital pressure. The symptoms suggest an orbital compartment syndrome, where the increased pressure within the orbit causes the eye to protrude (**proptosis**) and can lead to increased blood flow or congestion, resulting in a **hyperaemic sclera**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is not specified, but typically, conditions like orbital cellulitis could present with similar symptoms, however, the question hints more towards a mechanical increase in pressure rather than infection.
**Option B:** Similarly, this option is not specified but could be related to conditions that do not directly cause increased orbital pressure due to trauma.
**Option D:** This option, although unspecified, might relate to conditions that could cause proptosis or scleral hyperemia but not necessarily as a direct result of traumatic increase in orbital pressure.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **orbital compartment syndrome** is an emergency requiring prompt intervention to preserve vision. The **increased intraorbital pressure** can compromise the optic nerve and lead to permanent vision loss if not addressed quickly.

**Correct Answer:** D. Orbital compartment syndrome.