A heavy, thick-walled tube, usually steel, used between the drill pipe and the bit in the drill stem to weight the bit in order to improve its performance is called a __________.

Correct Answer: Option D — drill collar

Drill collars are heavy-walled, high-strength steel tubes placed at the bottom of the drill string. Their primary function is to provide the necessary weight on the bit to ensure efficient rock penetration and to keep the drill string in tension, which helps prevent buckling during the drilling process.