**Core Concept**
The most common primary brain tumor in adults is a type of glioma, specifically a tumor that arises from the glial cells of the central nervous system. This tumor type is known for its slow growth and variable clinical presentation.

**Why the Correct Answer is Right**
The correct answer is a glioblastoma, which is a type of astrocytoma. Glioblastomas are highly aggressive and malignant tumors that arise from the astrocytes, a type of glial cell in the brain. These tumors are characterized by their rapid growth and tendency to invade surrounding brain tissue. The pathogenesis of glioblastoma is complex and involves the activation of various signaling pathways, including the PI3K/AKT and RAS/MAPK pathways. The tumor microenvironment also plays a crucial role in the progression of glioblastoma, with the presence of cancer-associated fibroblasts and immune cells contributing to tumor growth and resistance to therapy.

**Why Each Wrong Option is Incorrect**
* **Option A:** Medulloblastoma is a type of brain tumor that is most commonly found in children, not adults.
* **Option B:** Ependymoma is a type of glioma that arises from the ependymal cells lining the ventricles and central canal of the spinal cord. While it is a type of glioma, it is not the most common type.
* **Option C:** Meningioma is a type of tumor that arises from the meninges, the protective membranes surrounding the brain and spinal cord. While it is a common type of brain tumor, it is not the most common type.

**Clinical Pearl / High-Yield Fact**
Glioblastoma is the most common primary brain tumor in adults and has a poor prognosis, with a median survival time of approximately 12-18 months despite aggressive treatment.

**Correct Answer:** C. Glioblastoma