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

A G0 (zero) constraint typically refers to a situation where two geometric entities, such as curves or edges, are contiguous but not tangent. This means that while the two objects touch each other at an endpoint or along a line, they do not share a common tangent leading to a smooth transition or curvature at the intersection. It is often used in design scenarios where you want to maintain proximity and alignment without ensuring that they meet smoothly.

This type of constraint allows for flexibility in design as it maintains a degree of separation between the objects, which can be beneficial depending on the specific requirements of the model or assembly. The other options describe conditions that either imply tangency or intersection with constraints that involve more specific geometric relationships, which do not align with the nature of a G0 constraint.