Which of these is a common file format for external data exchange?

XML, or Extensible Markup Language, is a widely used format for external data exchange due to its ability to store and transport data in a structured way that is both human-readable and machine-readable. It supports a variety of data types and can be easily parsed by different programming languages, making it ideal for interoperability between systems.

In contrast, DOCX and XLSX are file formats primarily associated with word processing and spreadsheet applications, respectively, and are not typically used for standard data exchange between systems. They are more focused on presentation and user interaction rather than pure data representation. PNG, on the other hand, is a raster graphics file format used for images and does not relate to structured data exchange at all. Therefore, XML stands out as the most suitable choice for external data exchange among the options given.