M8 command turns the ______ on.

The M8 command is used in G-code programming to activate the coolant system in a CNC machine. When a machining process involves cutting metals or other materials, coolant is essential to manage the temperature, reduce tool wear, and improve surface finish. By using the M8 command, the operator ensures that the coolant is flowing during the machining operation, which can help prevent overheating of both the workpiece and the cutting tool.

In contrast, the other options pertain to different functionalities in CNC operations. The spindle is typically controlled by separate commands designed for starting or stopping its rotation, feed rate is adjusted through specific settings or commands related to motion control, and tool selection or change is managed by distinct commands for tool changes.