Granting more permissions than necessary violates the principle of least privilege, a fundamental security best practice. While option A grants the necessary permissions (as subsets exist in two predefined roles), it might also grant more permissions than the Operations team strictly requires for their tasks on GKE and Cloud SQL.
Option D is too broad; 'Admin' roles grant extensive permissions that likely exceed the specific needs.
Google Cloud's best practices strongly recommend adhering to the principle of least privilege. Creating a custom role allows you to precisely define the set of permissions the Operations team needs for their specific tasks on the GKE cluster and the Cloud SQL instance, without granting any unnecessary permissions. This minimizes the potential blast radius in case of accidental or malicious actions.
Google Cloud Documentation References:
IAM best practices: https://cloud.google.com/iam/docs/best-practices - This document explicitly recommends granting the minimum necessary permissions.
Creating and managing custom roles: https://cloud.google.com/iam/docs/creating-managing-custom-roles - This explains how to create roles tailored to specific job functions.
Understanding roles: https://cloud.google.com/iam/docs/understanding-roles - This outlines the concepts of predefined and custom roles and their use cases.

https://cloud.google.com/spanner/docs/iam#spanner.databaseUser
Using the gcloud tool, execute the gcloud iam roles describe roles/spanner.databaseUser command on Cloud Shell. Attach the users to a newly created Google group and add the group to the role.

Forwarding zones Cloud DNS forwarding zones let you configure target name servers for specific private zones. Using a forwarding zone is one way to implement outbound DNS forwarding from your VPC network.
A Cloud DNS forwarding zone is a special type of Cloud DNS private zone. Instead of creating records within the zone, you specify a set of forwarding targets. Each forwarding target is an IP address of a DNS server, located in your VPC network, or in an on-premises network connected to your VPC network by Cloud VPN or Cloud Interconnect.
https://cloud.google.com/nat/docs/overview
DNS configuration Your on-premises network must have DNS zones and records configured so that Google domain names resolve to the set of IP addresses for either private.googleapis.com or restricted.googleapis.
com. You can create Cloud DNS managed private zones and use a Cloud DNS inbound server policy, or you can configure on-premises name servers. For example, you can use BIND or Microsoft Active Directory DNS.
https://cloud.google.com/vpc/docs/configure-private-google-access-hybrid#config-domain