Comprehensive and Detailed Explanation From FortiOS 7.6, FortiWeb 7.4 Exact Extract study guide:

According to theFortiOS 7.6 Azure Administration Guideand theFortinet 7.4 Public Cloud Securitydocumentation regarding Azure Virtual WAN (vWAN) architectures, the choice of connectivity depends on the required performance, security, and scale:

ExpressRoute (Option D):For a large-scale enterprise deployment involving acompany headquartersand multiplebranch sites, ExpressRoute is the "best" and most robust solution. It provides aprivate, dedicated, and high-throughput connection(up to 100 Gbps) that bypasses the public internet entirely. This ensures predictable low latency and higher reliability compared to internet-based tunnels.

Virtual WAN Integration:Azure vWAN Standard SKU explicitly supportsExpressRoute gatewaysas a primary connectivity method for on-premises sites. This allows the vWAN hub to act as a global transit point, seamlessly connecting the ExpressRoute-linked headquarters to other branch sites and VNET spokes.

Scalability for Headquarters:While site-to-site IPsec VPNs are common for smaller branches, the "main company site" or headquarters typically requires the high bandwidth and SLA guarantees provided byExpressRoute.

Why other options are incorrect:

Option A & B:L2TPandSSL VPNare primarily used for remote user access (Point-to-Site) rather than permanent site-to-hub infrastructure connections. vWAN uses OpenVPN or IKEv2 for user VPNs, not L2TP.

Option C:WhileGRE tunnelsare used in some networking scenarios, they are not a native, primary gateway connectivity option for the Azure vWAN hub compared to the standardized Site-to-Site VPN (IPsec) and ExpressRoute.