Which AWS service or instance purchasing option should the company use to meet this requirement?

Explanations:

EC2 Instance Savings Plans are designed for predictable workloads where you commit to using a certain amount of capacity for a period of time, but they do not automatically adjust the number of running instances based on demand.

Spot Instances allow you to bid for unused EC2 capacity at a lower price, but they are not suitable for applications with variable usage patterns as they can be terminated by AWS when the demand for capacity increases, leading to potential service disruptions.

Reserved Instances provide a significant discount for committing to a certain instance type and region for a term of 1 or 3 years, which is beneficial for predictable workloads but does not address the need for dynamic scaling based on usage.

Amazon EC2 Auto Scaling automatically adjusts the number of EC2 instances in response to the application load, ensuring that there are enough instances during high usage times and reducing the number of instances when usage is low, effectively optimizing costs.