Final answer:
The most common cause of neonatal mortality in twin pregnancies is preterm birth due to the associated complications, such as underdeveloped organ systems and Respiratory Distress Syndrome.
Step-by-step explanation:
The most common cause of neonatal mortality in twin pregnancies is Preterm birth (Option A). Premature birth can lead to Respiratory Distress Syndrome (RDS) and other significant health issues that increase the risk of mortality. While other factors such as Congenital anomalies, Intrauterine growth restriction (IUGR), and Twin-to-twin transfusion syndrome also contribute to neonatal mortality, the primary risk factor for neonatal death in twins is the complications arising from being born too early. These complications can include underdeveloped organ systems which can lead to a variety of health issues. IUGR may be due to placental insufficiency, and congenital anomalies can be due to genetic or environmental factors, but the association with increased mortality risk is not as strong as that with preterm birth.