The main natural causes of serious damage to Dunhuang murals are (a) flowing water erosion, (b) weathering and wind erosion, (c) glacier erosion and (d) water dissolution.

A gullies (v-shaped), waterfalls and canyons are often formed by flowing water erosion; Dunhuang is in the northwest of China, with little precipitation, dry climate and windy weather, so wind is the main factor. The main natural reason for the serious damage of Dunhuang murals is the result of wind, so it does not meet the meaning of the question.

B, wind erosion forms wind erosion mushrooms and wind erosion depressions; Dunes and ridges are formed by wind accumulation (generally, wind plays an obvious role in arid and semi-arid areas). Dunhuang is located in the northwest of China, with less precipitation, dry climate and windy weather. The main natural reasons for the serious damage of Dunhuang murals are the wind.

C. Glacier landforms are formed by glaciation, and there are often glacier U valleys, ridges, ice buckets and tinder. Dunhuang is in the northwest of China, with little precipitation, dry climate and windy weather, so it is dominated by wind. The main natural reason for the serious damage of Dunhuang murals is the result of wind, so it does not meet the meaning of the question;

D. karst landforms are formed by the dissolution of water. Dunhuang is in the northwest of China, with little precipitation, dry climate and windy weather, so wind is the main factor. The main natural reason for the serious damage of Dunhuang murals is the result of wind, so it does not meet the meaning of the question.

Therefore, choose: B.