The correct answer is option C. In Salesforce, a custom SOAP Web Service is defined using Apex with annotations that enable classes and methods to be exposed as web service endpoints. Helper methods that are not used by the Web Application should not be declared as webservice. They can be private or public but should not have the webservice keyword in their declaration. The webservice keyword is specifically for methods that are exposed to external applications via SOAP API.
References: The information can be referenced from the official Salesforce documentation on creating custom web services with Apex: Creating Custom Web Services.