A security engineer needs to protect a public web application that runs in a VPC.

Option D is the correct solution because it provides fully automated, real-time detection and mitigation of application-layer (Layer 7) DDoS attacks with no manual intervention. AWS Shield Advanced includes automatic application layer DDoS mitigation when it is enabled for supported resources such as Amazon CloudFront distributions. This feature continuously monitors traffic patterns and, when an attack is detected, automatically deploys AWS WAF rules to mitigate malicious requests.

Adding a rate-based rule to the AWS WAF web ACL further strengthens protection by automatically blocking IP addresses that exceed a defined request threshold, which is a common characteristic of Layer 7 DDoS attacks. This combination aligns directly with AWS best practices for protecting web applications against volumetric and application-layer threats.

Option A only provides alerting and visibility but does not ensure automated mitigation. Option B includes proactive engagement with the AWS DDoS Response Team, which is valuable for complex or large-scale attacks but still involves human interaction and therefore does not meet the “no manual effort” requirement. Option C introduces unnecessary complexity and is not recommended for protecting CloudFront-based applications against Layer 7 DDoS attacks.

AWS Security Specialty documentation explicitly recommends AWS Shield Advanced with automatic application layer DDoS mitigation and AWS WAF rate-based rules for fully automated, real-time protection of public web applications.