In FortiSwitchOS 7.6, achieving link-level redundancy and active-active uplink utilization acrosstwo separate FortiSwitch unitsrequires a technology that operates independently of Spanning Tree Protocol (STP). This requirement is fulfilled byMultichassis Link Aggregation Group (MCLAG).

MCLAG allows two FortiSwitch devices to operate as alogical aggregation peer, presenting themselves as a single logical switch to an upstream device such as a FortiGate. With MCLAG, FortiGate can form a singleLACP-based aggregated interfacethat spans both FortiSwitches. This enablessimultaneous use of uplinks on both switches, providing full bandwidth utilization and redundancy without blocking links, which is a fundamental limitation of STP-based designs.

According to the FortiSwitchOS 7.6 Administrator Guide, MCLAG synchronizes control-plane information between the two FortiSwitch peers using inter-switch links (ISLs) and dedicated keepalive mechanisms. This ensures consistent forwarding behavior and loop-free topology while allowing all member links to remain active. If one FortiSwitch fails, traffic continues to flow through the remaining switch with minimal disruption.

The other options do not meet the stated requirement. A standard LAG (Option D) operates only within asingle switchand cannot span multiple FortiSwitch units. Multi-tier topology (Option A) and full mesh HA (Option C) describe architectural layouts or FortiGate HA concepts but do not provide link-level aggregation across switches.

Therefore, the only configuration that allows FortiGate to use uplinks on both FortiSwitches simultaneouslywithout relying on STPisMultichassis Link Aggregation Group (MCLAG), makingOption Bthe correct and fully verified answer.