**Core Concept**
The question pertains to the most common type of eyeball tumor, which is often benign and affects the uveal tract, including the iris, choroid, and ciliary body. This tumor is a type of neoplasm that arises from the pigment epithelium of the iris.

**Why the Correct Answer is Right**
The correct answer is **Melanoma**. Melanoma is the most common primary intraocular malignancy in adults, accounting for approximately 5% of all ocular tumors. It arises from the melanocytes in the uveal tract, particularly in the choroid, and is often associated with a high degree of malignancy and poor prognosis if left untreated. The pathogenesis of melanoma involves mutations in the BAP1 gene, leading to uncontrolled cell proliferation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Retinoblastoma is a malignant tumor that primarily affects children and is the most common intraocular malignancy in this age group. It arises from the retina, not the uveal tract.
* **Option B:** Cavernous hemangioma is the most common benign orbital tumor, but it is not an eyeball tumor. It arises from the orbital soft tissues, not the eyeball itself.
* **Option C:** Astrocytoma is a type of glioma that can occur in the retina, but it is relatively rare and not the most common eyeball tumor.

**Clinical Pearl / High-Yield Fact**
Melanoma is often asymptomatic in the early stages, but patients may present with symptoms such as vision loss, floaters, or a change in the appearance of the iris.

**Correct Answer:** C.