**Core Concept**
Posterior mediastinal tumors arise from the tissues located within the posterior mediastinum, including the esophagus, nerves, and the vertebral column. These tumors can be benign or malignant and are often categorized based on their embryological origin.

**Why the Correct Answer is Right**
Thymomas, on the other hand, are typically anterior mediastinal tumors that arise from the epithelial cells of the thymus gland. The thymus gland is located in the anterior mediastinum and is responsible for the development and maturation of T-lymphocytes. Thymomas are often associated with autoimmune disorders such as myasthenia gravis.

**Why Each Wrong Option is Incorrect**
**Option A:** Neurofibroma can occur in the posterior mediastinum as part of a neurofibromatosis syndrome, which affects the neural crest cells that give rise to Schwann cells and melanocytes. Neurofibromas are benign nerve sheath tumors that can arise from the posterior mediastinal nerves.

**Option B:** Lymphoma can occur in the mediastinum, including the posterior mediastinum, and is a type of malignant tumor that arises from the lymphoid tissues. Lymphomas can arise from the lymph nodes, thymus, or other lymphoid tissues within the mediastinum.

**Option D:** Gastroenteric cysts, also known as enteric cysts, are benign tumors that arise from the remnants of the embryonic foregut and can occur in the posterior mediastinum. These cysts are usually congenital in origin and can contain mucinous or enteric-type tissue.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the location of mediastinal tumors can provide valuable clues about their etiology and potential diagnosis. For example, anterior mediastinal tumors are more likely to be thymomas or teratomas, while posterior mediastinal tumors are more likely to be neurogenic tumors such as neurofibromas or schwannomas.

**✓ Correct Answer: C. Thymoma**