What practice enhances security for virtual machines executing builds on Google Cloud?

Implementing private pools in Cloud Build with VPC service controls significantly enhances security for virtual machines executing builds on Google Cloud. Private pools allow you to run builds using dedicated virtual machines that are not exposed to the public internet, thus minimizing the attack surface.

In conjunction with VPC service controls, which provide an additional layer of security by allowing you to define a security perimeter around your Google Cloud resources, this practice ensures that sensitive data and resources remain protected against unauthorized access. This setup limits exposure by confining resources to specific network boundaries while providing a clear framework for controlling data access.

The other options do not offer the same level of security enhancement. Default pools may not provide the same isolation from the internet, and while utilizing Compute Engine VMs in a custom VPC can improve network configuration, it does not inherently enhance security like private pools can alongside VPC service controls. Finally, while employing Compute Engine Spot VM instances can be cost-effective, it does not focus on security as a primary consideration for builds.