**Core Concept**
A transilluminant tumor is a type of tumor that is characterized by its ability to allow light to pass through it when a light source is applied on the opposite side. This phenomenon occurs due to the presence of fluid-filled spaces within the tumor, which is a characteristic feature of certain types of neoplasms.

**Why the Correct Answer is Right**
The correct answer is a lymphoma, specifically a type of non-Hodgkin lymphoma. This is because lymphomas are composed of abnormal lymphocytes that accumulate in the lymphoid tissues, leading to the formation of fluid-filled spaces within the tumor. When a light source is applied on the opposite side of the neck, it can pass through these fluid-filled spaces, making the tumor transilluminant.

• **Option A:** A thyroid tumor is incorrect because thyroid tumors are typically solid and do not have fluid-filled spaces that would allow light to pass through.
• **Option B:** A cystic hygroma is incorrect because, although it is a fluid-filled lesion, it is not a neoplasm and does not exhibit the characteristic transillumination of a lymphoma.
• **Option C:** A branchial cleft cyst is incorrect because it is a congenital anomaly that is typically filled with a thick, cheesy material and does not exhibit transillumination.

**Clinical Pearl / High-Yield Fact**
The ability of a tumor to transilluminate is a classic clinical finding that can help differentiate lymphomas from other types of neck masses. This is an important distinction because lymphomas often require prompt treatment, whereas other types of tumors may not.

**Correct Answer: C. A lymphoma.**