score:5
The MARC-Penn commuter train on Sundays has an 11:15AM departure from BWI (via shuttle bus), arriving Bowie State Station at 11:28 or New Carrollton at 11:36. If their flight is on time they should make that without difficulty.
If their flight is late, the next departure is at 1:00PM, so they might want to consider Uber as a backup.
From Dulles they could take the Silver Line Express Bus to Wiehle-Reston Metro Station, and take the Silver Line and Orange Line to New Carrollton. This means two transfers but it's entirely inside the Metro system. It will be much slower (probably close to two hours) but service is more frequent.