The suggested answer is A, B, C.
To support the current growth plans while minimizing resources at the witness site, the administrator should first add the new vSAN witness appliance to vCenter Server. Next, they need to deploy a new large vSAN witness appliance at the witness site to handle the increased workload. Finally, the administrator should configure the vSAN stretched cluster to use the new vSAN witness appliance. These steps ensure that the expanded cluster is properly configured and utilizes the new witness appliance effectively, without the need for an extra-large witness or a shared witness appliance, which is not required for a single stretched cluster.