Refer to the exhibit. After this new OSPF design with per-packet load balancing was implemented, Host A reported that large file downloads from Server A became slow and sometimes failed. The operation

Upgrading all links to 10 Gb/s is not cost-conscious and may not eliminate ECMP if the upgraded links still compute equal OSPF costs.

Setting MTU to 1500 on all interfaces does not affect OSPF cost calculations or path selection and will not stop per-packet load balancing.

Adjusting the reference bandwidth only on R4 produces inconsistent cost calculations relative to the rest of the network, risking routing loops and suboptimal path selection without reliably eliminating ECMP.

Adjusting the auto-cost reference bandwidth on all routers recalculates interface costs consistently across the entire topology, which can break the cost equality that enables ECMP and per-packet load balancing. Without equal-cost paths, OSPF selects a single best path, ensuring packets traverse one route and arrive in order. The change must be applied to all routers to prevent inconsistent cost calculations that could cause routing loops or suboptimal forwarding.