You’re deploying test environments using Compute Engine VMs. Some downtime is acceptable, and it is very important to deploy them as inexpensively as possible. What single thing below could save you the most money?

This answer is correct. These are the most cost-effective solution as required in the question(up to 80% lower than equivalent non-preemptible machines) and the fact that some downtime is acceptable means that the requirement can be met with preemptible machines.