The correct options are
A This region consists of relic mountains.
B This region consists of old mountains that have undergone glacial erosion.
C This region consists of low mountains.
D Many rivers cross this region.
The Northwestern highlands consists of oldest mountains which are located in countries like Finland, Norway and Sweden. This region consists of relic mountains which have undergone glacial erosion and therefore became low mountains. Many rivers cross this region and it has a gentle slope as well in the north.