The correct answer is Four virtual machines.

The App Service plan is scaled out to four instances so there are four underlying worker virtual machines hosting the apps. Multiple web apps and their production and staging slots share those instances so neither the six apps nor the staging slots increase the number of virtual machines beyond the number of instances.

Twenty four virtual machines is incorrect because that option multiplies apps and slots and then assumes separate VMs per slot which is not how App Service scaling works.

Twelve virtual machines is incorrect because it assumes each app slot runs on its own VM. Deployment slots do not create separate VMs and the 12 slots are hosted on the four instances.

One virtual machine is incorrect because the plan is explicitly scaled out to four instances so there are four worker VMs rather than a single VM.

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