The correct answer is D: Define a design for the security of data in your web application that meets GDPR requirements. This is because while using Google Cloud Platform's native features (Option A) and enabling GDPR compliance settings in the GCP Console (Option B) can help, they are not sufficient on their own to ensure GDPR compliance. Similarly, incorporating Cloud Security Scanner in your test strategy (Option C) can identify compliance gaps but does not fully ensure GDPR compliance. As the technical architect, you need to design the security of data in your application to meet all GDPR requirements, which is why Option D is the correct choice.