**Core Concept**
The boy's condition is an ocular emergency requiring prompt evaluation and management. Hyphaema, a condition characterized by the accumulation of blood in the anterior chamber of the eye, can result from various causes including trauma, surgery, or spontaneous bleeding due to ocular disorders.

**Why the Correct Answer is Right**
The most likely source of blood in this case is the ciliary body, a structure located behind the iris that produces aqueous humor. The ciliary body is rich in blood vessels and is a common site for bleeding in cases of blunt trauma to the eye. When a tennis ball strikes the eye, it can cause a contusion to the ciliary body, leading to bleeding into the anterior chamber. This is a common mechanism of injury in sports-related eye trauma.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is likely referring to the iris, which is a less common source of bleeding in this context. While the iris can be damaged in cases of trauma, the ciliary body is a more likely source of bleeding in this scenario.
* **Option B:** This option is likely referring to the choroid, a layer of blood vessels between the sclera and retina. While the choroid can be a source of bleeding in cases of trauma, it is not the most likely source in this scenario.
* **Option D:** This option is likely referring to the lens, which is a less common source of bleeding in cases of trauma. The lens is not typically damaged in cases of blunt trauma to the eye.

**Clinical Pearl / High-Yield Fact**
In cases of blunt trauma to the eye, it is essential to carefully evaluate the patient for signs of hyphaema, as this can lead to complications such as increased intraocular pressure and vision loss.

**Correct Answer:** C.