**Core Concept**
Tennis racquet cells are a characteristic histopathological feature seen in a specific type of tumor. These cells are characterized by their appearance, which resembles the shape of a tennis racquet. This unique cellular morphology is a key diagnostic clue in identifying the underlying pathology.

**Why the Correct Answer is Right**
Rhabdomyosarcoma is a malignant tumor of skeletal muscle origin, and tennis racquet cells are a classic histopathological feature of this tumor. These cells are actually rhabdomyoblasts, which are immature skeletal muscle cells that exhibit a characteristic 'strap' or 'racquet' appearance under the microscope due to the presence of cross-striations. The presence of tennis racquet cells, along with other histopathological features, helps in the diagnosis of rhabdomyosarcoma.

**Why Each Wrong Option is Incorrect**
**Option A:** Rhabdomyoma is a benign tumor of skeletal muscle origin, and while it may exhibit some degree of cellular atypia, it does not typically display the characteristic tennis racquet cells seen in rhabdomyosarcoma.

**Option C:** Histiocytoma is a type of non-malignant tumor that arises from histiocytes, a type of immune cell. It does not exhibit the characteristic tennis racquet cells of rhabdomyosarcoma.

**Option D:** Eosinophilic granuloma is a type of non-malignant tumor that arises from the Langerhans cells of the immune system. It does not exhibit the characteristic tennis racquet cells of rhabdomyosarcoma.

**Clinical Pearl / High-Yield Fact**
Rhabdomyosarcoma is the most common primary malignant tumor of the orbit in children, and tennis racquet cells are a key diagnostic feature. Remember that the presence of tennis racquet cells, along with other histopathological features, is essential for the diagnosis of rhabdomyosarcoma.

**Correct Answer:**
✓ Correct Answer: B. Rhabdomyosarcoma