Which protocol is primarily used for handling XML-based messaging?

The correct choice is SOAP, which stands for Simple Object Access Protocol. SOAP is a protocol specifically designed for exchanging structured information in the form of XML messages over a network, particularly in the context of web services. It provides a standard messaging framework which ensures that the messages are sent between applications and are understood correctly, irrespective of the programming languages or platforms being used.

SOAP relies on XML as its message format and uses various transport protocols, like HTTP and SMTP, to facilitate communication between services. Its design incorporates features such as extensibility, neutrality, and independence, making it a robust choice for building distributed systems.

In contrast, SPML (Service Provisioning Markup Language) focuses on providing a protocol for the management and provisioning of identity-related services but does not serve as a general messaging protocol for XML. HTTP is a transport protocol that can carry a variety of data types, including XML messages, but it is not specific to XML-based messaging like SOAP is. XACML (eXtensible Access Control Markup Language) is designed for access control policies and is not used for handling XML-based messaging.

Overall, SOAP is the primary protocol that addresses the requirements of XML-based messaging, making it the most appropriate answer in this context.