Which of the following describes a G0 (zero) constraint?

A G0 (zero) constraint is characterized by the condition where objects are contiguous but not tangent. This means that while two geometrical entities may meet each other at a point (contiguous), they do not touch smoothly at that point, implying there could be a sharp corner or break in continuity at the junction. Such a constraint defines them being aligned or close together without any smooth transition or tangential contact, which is a fundamental aspect of defining relationships between geometrical elements in CAD and modeling software like Siemens NX.

In contrast, the other options describe different types of relationships between geometric elements. For instance, tangent relationships (as mentioned in the other choices) imply a smooth connection between two curves, requiring considerations of curvature which is not applicable in a G0 context. Understanding the nature of the G0 constraint provides essential insight into how CAD systems interpret and manage geometric relationships, ensuring designers can accurately represent their intended designs.