The axis of motion that is parallel to the spindle axis is always the Z-axis.
Explanation:
Z-Axis Which axis is which depends on the orientation of the spindle.
If the spindle is vertical (Figure 2.1), the Z-axis is vertical. Either the quill or the knee of a vertical spindle mill will move when a Z-axis command is executed.
This is the best answer I could give you, maybe you could show us a pic of the question so we see all the possible choices?
