To ensure an .mxd file can be opened in a previous ArcGIS version, one should use which command?

Using the Save As command allows users to maintain compatibility with earlier versions of ArcGIS when saving .mxd files. This command typically provides the option to choose the desired version of ArcGIS, which ensures that the saved file can be opened in that version or in later versions. This feature is particularly useful in collaborative environments where different users may have different versions of ArcGIS.

The other choices do not serve the purpose of saving an .mxd file in a format compatible with earlier versions of the software. Creating a Map Package is primarily focused on bundling map documents and their associated data for sharing but does not address version compatibility for .mxd files. Exporting a map might create graphical outputs like images or PDFs but does not produce a .mxd file at all. Saving a Copy could seem appropriate but typically does not allow for selecting a specific earlier version, limiting its utility for this purpose.