Which soil order is of major significance in the arid regions of Rajasthan?

Soils are grouped into orders under the USDA soil taxonomy mainly on the basis of their moisture regime and degree of development.

Rajasthan's western half is dominated by an arid to semi-arid desert climate with very low rainfall and high evaporation. Under such a dry (aridic) moisture regime, the soils that develop are Aridisols — mineral-rich, calcareous, low in organic matter, and dry for most of the year. They are widespread across the desert districts of Jaisalmer, Bikaner, Barmer and Jodhpur, which is why Aridisols are the order of major significance for arid Rajasthan.

The other orders form under different conditions: Vertisols need a clayey parent material with a strong wet season, while Inceptisols and Alfisols require the more humid, higher-rainfall settings that the desert lacks. Hence Aridisols is the correct choice.