Three virtual machines are in a single availability set. Attempting to resize one VM fails with an allocation error, and the resize must succeed. Which action should you take?

When you encounter an allocation error during a VM resize operation in an availability set, it means the current cluster lacks the necessary compute resources to accommodate the requested VM size. Stopping all VMs in the availability set allows Azure to deallocate them and then reallocate them to a different cluster that has the required resources. Stopping only one or two VMs might not free up enough resources or trigger a cluster change. Removing the VM from the availability set would defeat the purpose of using an availability set for high availability and would not resolve the underlying allocation issue.