To commence cutting threads with a metal lathe, you should engage the __________.

Correct Answer: Option D — split or half-nut

The split or half-nut is the mechanical device that engages the carriage directly with the lead screw. This engagement provides the precise, positive longitudinal feed necessary for cutting threads. It must be engaged and disengaged at specific intervals, guided by the thread-chasing dial, to ensure accurate thread pitch and alignment.