To lower the costs for a business-critical workload running on a fixed set of Compute Engine instances without any performance implications, the best option is to purchase Committed Use Discounts (Option A). Committed Use Discounts allow you to commit to using a certain amount of resources (vCPUs, memory, etc.) for a 1 or 3 year term in exchange for a significant discount, which is ideal for stable workloads with predictable resource needs. Migrating the instances to a Managed Instance Group (Option B) or creating an Unmanaged Instance Group (Option D) does not directly reduce costs but can improve management and scalability. Converting the instances to preemptible virtual machines (Option C) can significantly reduce costs but is not suitable for business-critical workloads due to the possibility of instances being terminated by Google with short notice.