Final answer:
In a Virtual Network, Network Security Group (NSG) rules should be used to ensure that a database server can communicate exclusively with a web server by controlling traffic.
Step-by-step explanation:
To ensure that a database server can only communicate with the web server on a Virtual Network, you should use Network Security Group (NSG) rules.
NSG rules allow you to control inbound and outbound traffic to network interfaces (NIC), VMs, and subnets. By setting up these rules, you can define which traffic can or cannot pass through the network. For instance, you would configure the NSG to allow incoming traffic to the database server only from the IP address of the web server, effectively blocking all other traffic.