The correct answer is C because it meets the requirement of protecting the application from potential application-level attacks. Google Cloud Armor security policies are sets of rules that match on attributes from Layer 3 to Layer 7 to protect externally facing applications1. Web application firewall (WAF) rules are predefined rules that detect and mitigate common web attacks such as cross-site scripting (XSS), SQL injection, remote file inclusion, and more2. By applying a Google Cloud Armor security policy with WAF rules to the backend service, you can filter out malicious requests before they reach your application.
Option A is incorrect because Cloud CDN is a content delivery network that caches static content at the edge of Google's network, but it does not provide any protection against application-level attacks3. Option B is incorrect because firewall rules are applied at the VPC network level, not at the load balancer level4. Firewall rules also only match on Layer 3 and 4 attributes, not on Layer 7 attributes that are relevant for application-level attacks4. Option D is incorrect because VPC Service Controls perimeter is a feature that helps you secure your data from unauthorized access by users outside your organization, but it does not protect your application from external attacks.
Reference:
Security policy overview | Google Cloud Armor
Web application firewall (WAF) rules | Google Cloud Armor
Cloud CDN overview | Google Cloud
Using firewall rules | VPC
[VPC Service Controls overview | Google Cloud]