Cloud Identity-Aware Proxy (IAP) is the optimal solution because it provides a centralized authentication layer for applications hosted on GCP, enabling secure internet access to resources that were previously restricted to the corporate network. IAP integrates with Google Sign-in and supports two-factor authentication, meeting the requirement for enhanced security. It allows fine-grained access control based on user identity and context, without requiring VPN connectivity. While Cloud VPN (option D) could provide network-level access, it does not offer the application-level authentication layer with built-in 2FA support that IAP provides. Cloud Armor (B) is a DDoS protection and web application firewall service, not an authentication service. Cloud Endpoints (C) is an API management platform, not designed for providing authentication to web applications in this context.