Parts of a country behind the coast or a river’s banks:

A. Swamps
B. Hinterland
C. Isthmuses
D. None of these

One word substitute is Hinterland.
Swamps: an area of low-lying, uncultivated ground where water collects.
Hinterland: the remote areas of a country away from the coast or the banks of major rivers.
Isthmus: a narrow strip of land with sea on either side, forming a link between two larger areas of land.