Final answer:
The feature you should use is b) dynamic provisioning, which allows for automatic allocation of VMs based on user requests in a virtual desktop infrastructure.
Step-by-step explanation:
The feature you should use to automatically allocate VMs when a user requests one in a virtual desktop infrastructure is dynamic provisioning. Dynamic provisioning allows for the automatic creation of virtual machines (VMs) based on user demand, ensuring that the necessary resources are allocated efficiently. When a user requests a VM, the system will automatically provision and allocate the required resources to fulfill the request.