Why Correct: Erg specifically denotes sandy deserts dominated by shifting sand dunes, covering at least 20% of the surface.
Distractor Analysis: Reg describes stony deserts with gravel-covered plains. Hamada refers to rocky plateaus with exposed bedrock. Bad land denotes heavily eroded terrain with intricate drainage patterns.
Takeaway: Desert classification includes Serir (pebble-covered deserts) and Wadi (dry river valleys in arid regions).