Which method supports secure data transmission via web services?

SOAP is the correct choice for supporting secure data transmission via web services. It is a protocol that defines a standardized way to exchange structured information in the implementation of web services. SOAP messages can be transmitted over various network protocols, such as HTTP or SMTP, and can be secured using WS-Security, which adds security features like authentication, encryption, and integrity to the message.

SOAP is especially suitable for secure data transmission because it allows the inclusion of security headers in the message to protect sensitive information during transmission. This makes it an ideal option for applications that require higher levels of security, such as those involving financial information or personal data.

While REST is another popular architecture for web services and can be secured using HTTPS, it does not inherently define a protocol for ensuring security features. WSDL, on the other hand, is a descriptive language used for defining the services provided by SOAP but does not itself handle data transmission or security. XML is a markup language used to encode documents but is not a method of secure transmission by itself.