The correct options are
A It has sandy and rocky soil.
C It is less fertile and thinly populated.
D Maize, rice and wheat are major crops.
The Sub-mountainous region is a cluster of mountain ranges of low heights. This region has sandy and rocky soil and hence is not as fertile as the plains. The region also receives high rainfall. It is thinly populated. Maize, rice and wheat are the main crops grown in this region.