The correct answer is C, Spot Instances. Spot Instances allow users to take advantage of unused EC2 capacity at a significantly lower cost compared to On-Demand pricing. They are ideal for fault-tolerant workloads that can be interrupted and resumed, as they can utilize checkpoints to save state. This makes them the most cost-effective option for the scenario described, where the company is already running fault-tolerant workloads with periodic checkpoints. While Capacity Reservations (A) can provide capacity guarantees at a lower cost than On-Demand, they do not offer the same level of savings as Spot Instances. Amazon Lightsail (B) simplifies web application hosting but may not be suitable for the company's existing workloads. Dedicated Hosts (D) provide physical server hosting for EC2 instances, which can be useful for compliance but do not offer the greatest cost savings.