**Core Concept**
The most common tumor in children is a neoplastic growth that arises from the cells and tissues of the pediatric population. Pediatric malignancies can be broadly classified into solid tumors (e.g., neuroblastoma, Wilms tumor) and hematological malignancies (e.g., acute lymphoblastic leukemia).

**Why the Correct Answer is Right**
The correct answer is neuroblastoma. Neuroblastoma is a type of extracranial solid tumor that arises from the neural crest cells, typically in the adrenal glands or along the sympathetic nervous system. This tumor is most commonly diagnosed in children under the age of 5 years, with a peak incidence around 2-3 years of age. The pathogenesis of neuroblastoma involves genetic mutations that lead to uncontrolled cell proliferation and tumor growth.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a tumor that is most common in children. Many tumors, such as breast cancer and lung cancer, are more common in adults.
**Option B:** This option is incorrect because while Wilms tumor is a common pediatric malignancy, it is not the most common tumor in children. Wilms tumor is a type of kidney cancer that typically affects children under the age of 7 years.
**Option C:** This option is incorrect because acute lymphoblastic leukemia (ALL) is a type of hematological malignancy, but it is not the most common tumor in children. While ALL is a common pediatric leukemia, neuroblastoma is more frequently diagnosed in children.

**Clinical Pearl / High-Yield Fact**
Neuroblastoma is often diagnosed incidentally on imaging studies performed for unrelated reasons, such as a chest X-ray or ultrasound. A high index of suspicion and awareness of this tumor's presentation are crucial for early diagnosis and treatment.

**Correct Answer: B. Wilms tumor.**