The base diameter of a helix is defined by what?

The base diameter of a helix is defined by a pre-existing circle. This means that when creating a helix in SolidWorks, you typically start the helix from an existing circular geometry, such as a circle sketch. The diameter of this base circle serves as the foundation for the helix since the helix is essentially a spiral that wraps around the axis determined by this base circle.

Starting from a pre-existing circle ensures that the helix has its desired base diameter and aligns correctly with the overall design. The other options do not directly determine the base diameter in the context of creating a helix. For instance, a custom-defined diameter implies a measurement that may not necessarily refer back to an existing circle, while the height of the helix and the length of the coil relate more to the vertical and longitudinal properties of the helix rather than its initial diameter.