Final answer:
Optimizing application code is a fine method for improving VM performance because it enhances the efficiency of the programs running within the VM without necessarily adding more resources.
Step-by-step explanation:
The question asks about a fine method for improving Virtual Machine (VM) performance. While adding more virtual CPUs (vCPUs) or RAM can improve performance, these are considered broad methods since they increase the general resources available to the VM. Increasing network bandwidth can also help, but it's more situational and only improves performance related to network activities.
The fine method among the options provided is D) Optimizing application code. This is because it addresses the efficiency of the actual workloads running within the VM. By optimizing the application code, you can reduce the resource requirements of the application, thus improving performance without the need to add additional resources.