Field-Level Security in Apex: The isAccessible() method in Apex is used to verify whether a user has the necessary permissions to view a particular field. This ensures that users see only fields they are permitted to access based on their profile or permission set.

Why Option B is Correct:

The issue where sales reps can view shipment amounts, even though this should be restricted to managers, indicates a failure to enforce field-level security in the custom Apex code.

By using Schema.sObjectField.isAccessible() checks, developers can ensure that the Apex class respects field-level security settings.

Why Others Are Incorrect:

Option A: The isShareable keyword does not exist in Apex. Sharing rules and object-level permissions should be explicitly respected in the code.

Option C: While runAs is used in test classes to simulate user permissions, it is not applicable to enforce permissions in the runtime logic.

For detailed reference, see Salesforce Security Documentation: https://help.salesforce.com/