Final answer:
The minimum number of virtual networks that must be included in the design is 6, with a separate network for each branch office and a dedicated network for Azure Bastion.
Step-by-step explanation:
To design a virtual network infrastructure that ensures Remote Desktop connections to virtual machines can be protected by using Azure Bastion, the minimum number of virtual networks that must be included in the design is 6.
Each branch office needs to be connected to the closest Azure region. Since there are five branch offices located on five different continents, a virtual network must be created for each branch office. Additionally, a separate virtual network should be created in the Azure region for deploying and managing the Azure Bastion service. This brings the total number of virtual networks to six.
By having separate virtual networks for each branch office and a dedicated virtual network for Azure Bastion, you can effectively connect each office to the closest Azure region while ensuring secure Remote Desktop connections.