A VM in a VPC with single‑stack subnets needs a fixed IP so other services in the same VPC can reliably reach it. Follow Google‑recommended practices and minimize cost. What do you do?

The correct answer is to promote the VM's current internal address to a reserved static internal IP. This is the Google-recommended practice for ensuring other services within the same VPC can reliably reach the VM. By promoting the existing internal IP, you maintain the current network configuration while making the IP address persistent across VM restarts or recreations. This approach is cost-effective because internal IP addresses are free. Promoting the VM's current address to a static external IPv4 address is incorrect because the requirement is for other services in the same VPC to reach it, implying internal communication. Using an external IP for internal communication is unnecessary, less secure, and incurs additional cost. Reserving a new static external IPv6 or IPv4 address and assigning it to the VM is also incorrect for similar reasons; external IPs are not needed for internal VPC communication and incur cost. Furthermore, the question specifies single-stack subnets, which typically implies IPv4 unless IPv6 is explicitly mentioned as being enabled.