The correct answer is A. a service tag.

In order to ensure that the virtual machines can access Vault1 while also using the principle of least privilege and minimizing administrative effort, you should configure a service tag as the destination of the outbound security rule for NSG1. Service tags represent a group of IP addresses associated with Azure PaaS and SaaS services. By specifying a service tag as the destination of the outbound security rule, you can allow the virtual machines to access Vault1 without having to manually specify the IP addresses of Vault1. This reduces administrative effort and ensures that the virtual machines are only able to access Vault1, rather than any other internet destination.