Azure Virtual Machine Scale Sets (VMSS) are the most suitable option for this scenario because they allow your virtual machines to scale in or out automatically based on predefined rules and metrics, such as CPU usage or network traffic. This service provides high availability and ensures that the application can handle variable loads without manual intervention. Azure Virtual Machines are not the best option here because they don't automatically scale. Availability sets are used to improve the availability of individual virtual machines, but they don't provide auto-scaling capabilities. Lastly, Azure Virtual Desktop is a desktop virtualization service and is not used for scalable web application hosting.