**Core Concept**
Neonatal abdominal masses can be caused by various factors, including congenital anomalies, infections, and tumors. The most common abdominal mass in neonates is often related to a specific congenital condition that affects the urinary system.

**Why the Correct Answer is Right**
The commonest abdominal mass in neonates is typically a nephroblastoma (Wilms tumor) or a hydronephrotic kidney. Nephroblastoma is a type of kidney cancer that occurs in infants and young children, often presenting as an abdominal mass. The tumor arises from the metanephric blastema, a group of immature cells in the developing kidney. The mass can cause hydronephrosis due to urinary obstruction, leading to abdominal distension and discomfort.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a common cause of abdominal mass in neonates. While it may be a possible cause, it is not the most common one.
**Option B:** This option is incorrect because it is a rare condition in neonates. Meconium peritonum is more commonly associated with premature infants and may present as a bowel obstruction or perforation.
**Option C:** This option is incorrect because it is not a specific type of abdominal mass. Ovarian cysts can occur in neonates, but they are not the most common cause of abdominal mass.

**Clinical Pearl / High-Yield Fact**
In neonates, a palpable abdominal mass often warrants further investigation to rule out life-threatening conditions such as Wilms tumor or congenital anomalies.

**Correct Answer:** C.