The requirement involves enforcing a business rule across two different objects (Contact and Survey_Response__c) and through different entry points: standard UI/Triggers for Contacts and a custom Visualforce page for Survey Responses. Implementing the logic separately in a trigger and a controller (Option C) is a poor architectural choice because it duplicates code, leading to increased maintenance effort and a higher risk of inconsistency if the logic needs to change in the future. Validation rules (Option A) are also unsuitable here because they cannot natively perform a dynamic lookup against a list of records in a separate "Blocked Domains" object without complex and unscalable workarounds.
The optimal approach is to centralize the validation logic within a single "Helper" or "Service" class. This follows the DRY (Don't Repeat Yourself) principle of software development. By creating a shared method in a helper class, the developer can ensure that both the Contact trigger and the Visualforce controller use the exact same logic to verify email domains. This centralized method queries the custom object containing the blocked domains and returns a validation result. This architecture simplifies testing, ensures uniform enforcement of business rules across the entire platform, and allows administrators to update the blocked domains list without requiring any further code modifications.