**Core Concept:** Testicular tumours are rare in children, but the most common type is germ cell tumour. Germ cell tumors are derived from the germ cells, which are the precursor cells to sperm and eggs.

**Why the Correct Answer is Right:** The correct answer, **C. Seminoma**, is a type of germ cell tumor that originates from the germ cells in the testes. Seminomas are generally slow-growing and less aggressive compared to other types of germ cell tumors like **D. Teratoma** and **A. Mixed Germ Cell Tumor**, which include both germ cell components and other cell types. **B. Non-Germ Cell Tumors** are less common in children and account for a smaller proportion of testicular tumors.

**Why Each Wrong Option is Incorrect:**

A. **Mixed Germ Cell Tumor (Option A)**: This term refers to a tumor containing germ cell components and other cell types. Although germ cell tumors are common, mixed germ cell tumors are less common in children and less likely to be the commonest infantile testicular tumor.

B. **Non-Germ Cell Tumors (Option B)**: These are less common in children and comprise a smaller proportion of testicular tumors.

C. **Seminoma (Correct Answer)**: Seminoma is a type of germ cell tumor, which is the most common type of testicular tumor in infants and children. It is a slow-growing tumor, and the correct answer.

**Clinical Pearl:** Early diagnosis and treatment of testicular tumors are crucial, as they can have a high cure rate when detected early. Testicular self-examination should be encouraged for males, especially during puberty and adolescence, to identify any abnormal growth or lumps. If a testicular tumor is suspected, prompt consultation with a healthcare professional is necessary for further evaluation and management.