When integrating applications that require access to sensitive data stored in Cloud Storage, managing service account keys securely is crucial to prevent unauthorized access or data loss.​

Option A: Defining a VPC Service Controls perimeter enhances security by restricting access to Google Cloud services. However, configuring ingress rules to allow external access for the service account may introduce complexities and potential security gaps, especially if the partner's infrastructure is outside the defined perimeter.​

Option B: Scanning and masking customer data addresses data sensitivity but does not mitigate risks associated with compromised service account keys. This approach focuses on data content rather than access control mechanisms.​

Option C: Encrypting data at rest using customer-managed encryption keys (CMEK) ensures data confidentiality but does not directly address the security of service account keys or access controls.​

Option D: Implementing a secret management service to handle service account keys is a best practice. By configuring the service to frequently rotate keys, you reduce the window of opportunity for malicious actors to exploit compromised keys. Additionally, enforcing strict access controls ensures that only authorized personnel can create or manage service account keys, minimizing the risk of unauthorized access. This approach directly addresses the security concerns related to service account key management.​

Therefore, Option D is the most appropriate recommendation, as it focuses on securely managing service account keys through rotation and access controls, thereby minimizing the risk of data loss due to compromised keys.​