**Core Concept**
Optic nerve gliomas are a type of glial tumor that can affect the optic nerve, leading to vision loss and other symptoms. They are more common in children and can be associated with neurofibromatosis type 1 (NF1).

**Why the Correct Answer is Right**
Pilocytic astrocytomas are the most common type of optic nerve glioma, accounting for approximately 80% of cases. These tumors are typically low-grade (WHO Grade I) and have a relatively favorable prognosis with appropriate treatment. They are characterized by a biphasic pattern of compact, pilot-like bipolar cells and loose, microcystic areas. Pilocytic astrocytomas often occur in the optic nerve, but can also be found in other locations such as the cerebellum and brainstem.

**Why Each Wrong Option is Incorrect**
**Option A:** Gemistocytic astrocytomas are a rare type of glioma that typically occur in adults and are associated with a more aggressive clinical course. They are not the most common type of optic nerve glioma.
**Option B:** Fibrous astrocytomas are not a recognized classification of glioma, and fibrous tissue is not a characteristic feature of optic nerve gliomas.
**Option C:** Protoplasmic astrocytomas are a type of low-grade glioma, but they are not the most common type of optic nerve glioma. They are more commonly found in the cerebral cortex.

**Clinical Pearl / High-Yield Fact**
Pilocytic astrocytomas are often associated with a characteristic "target sign" on imaging, which can be helpful for diagnosis. This sign refers to a central area of low signal intensity surrounded by a ring of high signal intensity on T2-weighted MRI scans.

**✓ Correct Answer: D. Pilocytic**