Which default Reference Sets are available if no additional reference sets have been defined?

The correct option is "Entire Part," as it represents the default reference set available in Siemens NX when no additional reference sets have been defined. The Entire Part reference set typically encompasses all the features and geometry within the model. This set allows users to work with the complete structure of the part, ensuring that they have full visibility and access to all components of the design.

In terms of the other options, while "Empty" might suggest a reference set that contains no features, it does not serve practical needs for modeling since it would not allow any access to the part's geometry. "Lightweight" refers to a display mode that reduces the complexity of the model but is not a reference set itself. The option "All" implies a total inclusion of everything, which could lead to confusion as the term 'Entire Part' is more specific. Understanding the distinction between these terms is crucial for effectively using Siemens NX and managing reference sets in your modeling workflow.