Which of the following is a requirement for digital geospatial data according to ISO 19139?

The correct choice highlights that the storage of metadata in XML format is a key requirement for digital geospatial data according to ISO 19139. This standard provides guidelines for the encoding and content of geographic information and emphasizes the importance of structured metadata. Metadata in XML format allows for a standardized way of describing geospatial information, including details about the data's source, quality, lineage, and spatial reference. This structured format facilitates interoperability among geospatial datasets and supports the effective sharing and use of geographic information across different platforms and applications.

While other options might seem relevant, ISO 19139 does not mandate the exclusive use of text-based file formats, the involvement of independent review boards for approval, or the requirement for visual aids in documentation. Instead, the focus is on utilizing widely accepted standards that ensure data consistency, interoperability, and accessibility, which XML significantly contributes to when it comes to geospatial metadata.