**Core Concept**
Retinoblastoma is a malignant tumor of the retina, primarily affecting children. It arises from the immature retinal cells and can lead to vision loss, pain, and even death if left untreated. The commonest manifestation of retinoblastoma is related to the way it affects the eye and its surrounding structures.

**Why the Correct Answer is Right**
The commonest manifestation of retinoblastoma is leukocoria, which is an abnormal white reflection from the retina of the eye. This occurs due to the accumulation of calcium in the tumor cells, which reflects light and gives the appearance of a white pupil. Leukocoria is often the first sign of retinoblastoma and can be detected during routine eye examinations or photography. The tumor can also cause strabismus, glaucoma, and proptosis, but leukocoria is the most common presentation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Strabismus is not the commonest manifestation of retinoblastoma, although it can occur due to the tumor's effect on the eye muscles.
* **Option B:** Glaucoma is a complication of retinoblastoma, but it is not the most common presentation.
* **Option C:** Proptosis is a rare manifestation of retinoblastoma, and it is not the commonest presentation.

**Clinical Pearl / High-Yield Fact**
Retinoblastoma can be detected during routine eye examinations or photography, making early detection crucial for treatment and survival. A white pupil or leukocoria in a child should always raise suspicion for retinoblastoma.

**Correct Answer: A. Leukocoria**