Which default reference set is available if no additional reference sets have been defined?

The option indicating "Entire Part" as the default reference set is correct because this reference set represents the entire geometry of the part, including all features and elements, when no additional reference sets have been defined. In NX, the default reference sets are created to provide users with an initial framework for working with parts and assemblies, ensuring that they have access to all the necessary data for design and modeling.

When you select "Entire Part," you are essentially choosing the most comprehensive view of the part, which is important for accurate modifications, analysis, and downstream applications such as manufacturing. This reference set is fundamental, allowing users to access everything in the part without concern for any exclusions or modifications that might exist in additional customized reference sets.

The other options do not represent the comprehensive aspect that the "Entire Part" does. For instance, "Complete Part" might imply all features, but it is not a standard term used in NX for the default reference set, whereas "Main Assembly" typically refers to a specific assembly context rather than an individual part's geometry. "Base Model" may hold significance in some contexts, but it does not capture the same complete representation that "Entire Part" does when initially defining the reference sets.