The @TestVisible annotation allows test methods to access private or protected members of another class, such as methods, member variables, and inner classes. This is the standard and recommended practice for unit testing private methods without altering the method's intended visibility for non-test contexts. It provides necessary access for testing while maintaining proper encapsulation in the production code.

A. Use the SeeAllData annotation: This annotation provides access to org data, not private methods, and is generally discouraged as it hinders test data isolation.

B. Add a test method in the Apex class: Test methods must reside in a separate class annotated with @isTest, not within the class being tested.

C. Mark the Apex class as global: This unnecessarily exposes the entire class and its methods to all Apex code, which is a poor security and design practice.

- Salesforce Apex Developer Guide, "Testing Private Methods," which states, "To enable this, add the @TestVisible annotation to the private or protected members."

- Salesforce Apex Developer Guide, "Access Modifiers," which details the scope of private, protected, and global access modifiers.

- Salesforce Apex Developer Guide, "Isolation of Test Data from Organization Data in Unit Tests," which explains the purpose and drawbacks of using (SeeAllData=true).