To implement a custom SOAP web service in Apex, the class must be declared with the global access modifier. Methods that are intended to be exposed to external applications via the WSDL must be declared as both global and static, and they must use the webservice keyword.

Helper methods, which are for internal use within the class and should not be exposed in the WSDL, must be defined without the webservice keyword. Option D correctly follows this structure by defining a global class, a webservice global static method for external access, and a public helper method that remains internal to the Salesforce organization.

1. Salesforce Apex Developer Guide

"Apex Web Services":

Section: Defining Apex Web Service Methods: "To expose an Apex method as a web service

the method must be global static and be part of a global class. The webservice keyword must be used for any method that should be exposed." This directly supports why Option D is correct and why options A and C are incorrect.

Section: webservice Keyword: This section details the syntax and requirements

reinforcing that a webservice method must be global and static within a global class for SOAP services. This confirms why Option B is incorrect

as it lacks the necessary keyword for exposure.

2. Trailhead by Salesforce

"Apex Integration Services" Module:

Unit: Apex Web Services: "To expose an Apex class and methods so that external applications can access your code and your application

you must use the webservice and global keywords." This module provides examples that align with the structure shown in Option D

where the class is global and the exposed method is webservice static. It clarifies that methods without the webservice keyword are not exposed.