Methods commonly employed for drivage of gallens of coal are . . . . . . . .

Answer: Option D

Definition: Drivage of galleries in coal mining refers to the process of making underground roadways or passages to provide access to coal seams for extraction and transportation.

Common methods: Coal cutting machines and drills: Used to cut and break the coal in preparation for loading and transport. Heading machines, continuous miners, augers, and road headers: Mechanized equipment that simultaneously cuts, loads, and transports coal, increasing efficiency. Coal drills and tubs: Traditional methods involving drilling holes for blasting and using tubs for transporting the broken coal.

Reason: Different mining conditions, seam thicknesses, and production requirements may call for any of these methods, and all are recognized as standard practices in coal gallery drivage.

Conclusion: Since all the listed methods are commonly employed, the correct answer is All of these.