For a company using reserved EC2 instances across multiple units, what is the most cost-optimal solution to manage under-utilization and increased demand?

Identify Under-utilization: The first step is to identify under-utilized EC2 instances. AWS provides tools like AWS Cost Explorer and AWS Trusted Advisor which can help identify under-utilized instances.

Consolidate Workloads: Once under-utilized instances are identified, the next step is to consolidate workloads. This means combining workloads from multiple under-utilized instances to fewer instances. This can help in reducing the number of instances and hence the cost.

Use Auto Scaling: AWS Auto Scaling can be used to manage increased demand. Auto Scaling automatically adjusts the number of EC2 instances in response to changing demand patterns. This ensures that the number of EC2 instances is always optimal.

Use Reserved Instances for Predictable Workloads: Reserved instances provide a significant discount compared to On-Demand instances. Therefore, for predictable workloads, reserved instances should be used.

Use Spot Instances for Unpredictable Workloads: Spot instances offer even greater discounts compared to Reserved instances. However, they are not always available and can be interrupted by AWS. Therefore, they are suitable for workloads that are not time-sensitive and can tolerate interruptions.

Use Savings Plans: AWS Savings Plans is a flexible pricing model that offers significant savings on AWS compute usage. With Savings Plans, you commit to a consistent amount of usage (measured in $/hour) for a 1 or 3 year term, and in return, you receive a discount off the on-demand price.

Regularly Review and Optimize: Finally, it's important to regularly review and optimize the usage of EC2 instances. AWS provides various tools and reports that can help in this process.