**Core Concept**
Childhood malignancies can cause proptosis due to orbital invasion or metastasis. Proptosis refers to the forward displacement of the globe of the eye, which can result from various conditions, including tumors that originate within the orbit or metastasize to the orbit.

**Why the Correct Answer is Right**
Rhabdomyosarcoma is the most common primary orbital malignancy in children. It arises from the embryonal muscles of the orbit and can cause proptosis due to its aggressive growth pattern. The tumor produces a mass effect, leading to displacement of the eyeball forward. Rhabdomyosarcoma also has a tendency to invade surrounding structures, including the eyelids, lacrimal gland, and optic nerve.

**Why Each Wrong Option is Incorrect**
* **Option A:** Leukemia is a hematological malignancy that can cause proptosis, but it is not the most common childhood malignancy producing proptosis. Leukemia can cause proptosis due to leukemic infiltration of the orbit, but this is less common than primary orbital tumors like rhabdomyosarcoma.
* **Option B:** Neuroblastoma is a malignancy of the sympathetic nervous system that can produce proptosis in children. However, it is not the most common childhood malignancy producing proptosis. Neuroblastoma can metastasize to the orbit, causing proptosis, but it is less common than primary orbital tumors like rhabdomyosarcoma.
* **Option C:** Lymphoma is a malignancy of the immune system that can produce proptosis in children. However, it is not the most common childhood malignancy producing proptosis. Lymphoma can cause proptosis due to orbital involvement, but it is less common than primary orbital tumors like rhabdomyosarcoma.

**Clinical Pearl / High-Yield Fact**
Rhabdomyosarcoma is the most common primary orbital malignancy in children and can cause proptosis due to its aggressive growth pattern.

**Correct Answer:** C. Lymphoma