What defines the structure of messages in SOAP?

The correct choice highlights that the structure of messages in SOAP (Simple Object Access Protocol) is defined by its basis in XML (Extensible Markup Language). SOAP is designed to facilitate communication between applications over the internet, and it does so by using XML to encode the messages. This means that all data exchanged in a SOAP message follows a specific XML structure, which allows for easy parsing and understanding of the message content by software programs.

When a SOAP message is constructed, it includes various elements such as the envelope, header, and body, which must conform to the XML syntax rules. This use of XML ensures that messages are both human-readable and machine-readable, and that they can be transported across different platforms seamlessly.

The other alternatives do not accurately describe the defining characteristic of SOAP message structure. While data types and API rules are relevant in the context of web services, they do not specifically define the structural format used in SOAP messages. Similarly, the approval process flow is unrelated to the technical specifications of how SOAP messages are formatted and transmitted.