To reduce the amount of traffic sent over the ISL between Switch-1 and Switch-2, enable active forwarding on LAG 100 on both Switch-1 and Switch-2 is a correct explanation of how to control how core-to-access traffic is forwarded. Switch-1, Switch-2, and the router run OSPF on LAG
100, which is a Layer 3 LAG. Active forwarding is a feature that allows a switch to select one link as active and one link as standby for each direction of traffic in a LAG. Enabling active forwarding on LAG 100 on both Switch-1 and Switch-2 would reduce the amount of traffic sent over the ISL by sending traffic over only one link instead of both.