Duodenal atresia is frequently associated with Down syndrome, and is caused when the duodenum fails to recanalize during development. The infant presents with bilious vomiting, so we know that food is at least making it down to the duodenum where the pancreatic duct empties into the duodenum. This eliminates D (would present with nonbilious vomiting, typically a few weeks after birth) and E (would present with choking and vomiting immediately on feeding).

The presence of bile tells us that bile is getting to the duodenum, so extrahepatic biliary atresia does not seem possible (B). There is nothing to indicate that the child has an omphalocele (C) since the abdomen is distended with food that isn't passing, but the guts are still inside the abdominal cavity.

Since there were "small amounts of meconium" I thought it couldn't be atresia. Turns out atresia isn't always absence of lumen, it can also be abnormal narrowing of lumen, allowing just a small amount to pass through...