What element of FortiGate enables secure communication between remote sales offices and the main branch?

The selection of Site-to-site VPN as the correct answer is appropriate for enabling secure communication between remote sales offices and the main branch. A Site-to-site VPN establishes a secure and encrypted connection over the internet between two networks, allowing multiple users from different locations to access resources securely as if they were on the same local network. This is particularly beneficial for connecting entire offices or branch locations directly with the main office, ensuring that data transmitted between them remains confidential and protected from unauthorized access.

While SSL VPNs provide secure remote access for individual users connecting to a network, they are not specifically designed for permanent network-to-network connections. IPSec tunnels are a protocol used to establish a Site-to-site VPN and ensure encryption and security but do not define the complete solution in terms of connecting entire sites. Dynamic DNS helps to maintain accessibility to devices whose IP addresses may change, but it does not directly facilitate secure communications like a VPN does. Thus, the Site-to-site VPN is the most comprehensive solution among the choices for connecting distant offices securely to a central branch.