Which one of the following is the most common cause of congenital hydrocephalus is?

Hydrocephalus most commonly in children because of abnormality of the aqueductal stenosis or obstruction of the 4th ventricles. Aqueductal stenosis results from an abnormally narrow aqueduct of sylvius that is often associated with branching or forking. Ref: Kligman, Behrman, Jenson, Stanton (2008), Chapter 592, "Congenital Anomalies of the Central Nervous System", In the book, "Nelson's Textbook of Pediatrics", Volume 2, 18th Edition, New Delhi, Page 2453