What is a common issue when extruding from sketches that contain unconnected curves?

When extruding from sketches, having unconnected curves is problematic primarily because it can lead to failed extrusions. This is due to the fact that a valid closed profile is essential for the extrusion operation to take place successfully. If the curves are not connected, NX cannot determine a complete boundary to perform the extrusion, which results in a failure of the operation. Ensuring the sketch forms a closed loop is crucial for the effective generation of solid geometry.

While other options might imply issues related to sketching or modeling in general, they do not accurately capture the immediate impact of unconnected curves on the extrusion process. The inability to select curves, complexity of the model, or automatic fixing of the sketch do not directly address the core issue that arises specifically during the extrusion action, which is fundamentally reliant on having a closed and valid sketch.