**Core Concept**
A Colles fracture is a type of fracture that occurs at the distal end of the radius bone in the forearm, typically resulting from a fall onto an outstretched hand. The fracture often leads to a deformity, known as a "dinner fork deformity," due to the displacement of the bone fragments.

**Why the Correct Answer is Right**
The late complication of a Colles fracture is **malunion with carpal tunnel syndrome**. This occurs when the fracture heals in an improper position, causing compression of the median nerve in the carpal tunnel. The displacement of the bone fragments can lead to a decrease in the diameter of the carpal tunnel, resulting in nerve compression and symptoms such as numbness, tingling, and weakness in the hand.

**Why Each Wrong Option is Incorrect**
* **Option A:** Delayed union is a possible complication of a Colles fracture, but it is not the most common late complication. Delayed union occurs when the fracture takes longer than expected to heal, but it does not typically lead to nerve compression.
* **Option B:** Infection is a potential complication of any fracture, but it is not a specific late complication of a Colles fracture.
* **Option D:** Nonunion is a rare complication of a Colles fracture, but it is not the most common late complication.

**Clinical Pearl / High-Yield Fact**
It is essential to carefully monitor patients with Colles fractures for signs of malunion and carpal tunnel syndrome, as early treatment can prevent long-term nerve damage.

**Correct Answer:** C.