Your Dataproc cluster runs in a single Virtual Private Cloud (VPC) network in a single subnetwork with range 172.16.20.128/25. There are no private IP addresses available in the subnetwork. You want…

Modifying the existing subnetwork's IP range from 172.16.20.128/25 to 172.16.20.0/24 effectively expands the address space, providing more available private IP addresses within the same subnet, allowing new VMs to be added without creating new networks or peering, which is the simplest approach.

Secondary IP ranges are primarily for alias IPs on instances, not for expanding the primary IP range for new instances in the subnet. While possible, it's more complex than simply expanding the primary range and might not be suitable for primary IP assignment for new VMs.

Creating a new VPC network and then using VPC Peering involves multiple steps (network creation, subnet creation, peering configuration, routing setup) and adds complexity, which goes against the goal of minimizing steps.

Similar to option C, creating an entirely new VPC network and enabling VPC network peering is a more complex solution than simply expanding the existing subnetwork, introducing additional overhead and configuration steps.