What access restriction would not ensure web apps only allow traffic through Azure Front Door?

The correct answer is that allowing access restrictions based solely on backend IP of the Front Door instance would not ensure that web apps only allow traffic through Azure Front Door. This is because relying on the backend IP addresses alone does not provide a comprehensive security measure. The backend IP addresses can change, and there is the potential for other traffic to reach the application directly, bypassing Azure Front Door.

Using recognized Front Door service tags, on the other hand, helps to precisely define which Azure services can send traffic to your web applications. This method is reliable as it accounts for the dynamic nature of IP addresses and consistently provides updates directly from Microsoft regarding the IP ranges of the service tags.

Configuring network security rules adds an additional layer of control by defining allowed and denied traffic based on set parameters and policies, ensuring greater security for the resources. Finally, utilizing an application gateway can effectively manage and filter traffic, ensuring that only allowed traffic is processed, making your web applications more secure.

In essence, while relying on backend IPs might work under certain circumstances, it does not provide the robust, adaptable security needed to ensure that all traffic flows through Azure Front Door reliably.