Correct Answer: BTo enable virtual machines on VNET1 and VNET2 to communicate through the Microsoft backbone infrastructure, virtual network peering is the appropriate solution. Virtual network peering allows two virtual networks in different Azure regions to seamlessly connect and route traffic privately using Microsoft's backbone infrastructure. This ensures the communication between the virtual networks is secure and managed without needing the internet. Other options, such as Azure ExpressRoute, site-to-site VPN, and point-to-site VPN, are not necessary or ideal for this scenario as they either involve connecting to on-premises networks or are used for different types of connectivity requirements.