Explanation
Azure policies can be used to define requirements for resource properties during deployment and for already existing resources. Azure Policy controls properties such as the types or locations of resources.
Azure Policy is a service in Azure that you use to create, assign, and manage policies. These policies enforce different rules and effects over your resources, so those resources stay compliant with your corporate standards and service level agreements.
In this question, we would create a Azure policy assigned to the resource group that denies the creation of virtual machines in the resource group.
You could place a read-only lock on the resource group. However, that would prevent the creation of any resources in the resource group, not virtual machines only. Therefore, an Azure Policy is a better solution.
References:
https://docs.microsoft.com/en-us/azure/governance/policy/overview

Explanation

Section: Understand Azure Pricing and Support

Explanation:
Box 1: No
Most services go to private preview then public preview before being released to general availability. The private preview is only available to certain Azure customers for evaluation purposes.
Box 2: Yes
Public Preview means that the service is in public beta and can be tried out by anyone with an Azure subscription. Services in public preview are often offered at a discount price.
Public previews are excluded from SLAs and in some cases, no support is offered.
Box 3: No
An Azure service in general availability is available to all Azure customers, not just a subset of the customers.
References:
https://azure-overview.com/Home/Faq