**Core Concept**
The question is testing the knowledge of extramedullary relapse in Acute Lymphoblastic Leukemia (ALL). Extramedullary relapse occurs when the leukemia cells infiltrate and proliferate outside the bone marrow, often in the central nervous system (CNS) or other organs. This phenomenon is a significant clinical concern in the management of ALL.

**Why the Correct Answer is Right**
The commonest site of extramedullary relapse in ALL is the **testes**. This is because the testes are a sanctuary site for leukemia cells, providing a microenvironment that allows them to evade chemotherapy and other treatments. The testes are also a common site for testicular relapse in various types of leukemia, including ALL. The testicular involvement is often bilateral and can be accompanied by elevated tumor markers such as beta-hCG.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because while the CNS is a common site for extramedullary relapse in ALL, it is not the most common site.
* **Option B:** This option is incorrect because the liver and spleen are not typically considered primary sites for extramedullary relapse in ALL.
* **Option C:** This option is incorrect because the lungs are not a common site for extramedullary relapse in ALL.

**Clinical Pearl / High-Yield Fact**
The testes are a common site for extramedullary relapse in ALL, and regular testicular examinations are essential in the follow-up of patients with ALL.

**Correct Answer:** C.