Amazon EC2 On-Demand Instances are ideal for unpredictable workloads that do not require a long-term commitment. This option allows users to pay for compute capacity by the hour or second, with no long-term commitments. It is suitable for short-term, spiky, or unpredictable workloads that cannot be interrupted and require flexibility. Option A is better suited for Reserved Instances, Option B is ideal for Spot Instances, and Option D is more suited for Reserved Instances due to cost optimization for long-term use.

"There is no long-term commitment required when you purchase On-Demand Instances. You pay only for the seconds that your On-Demand Instances are in the running state, with a 60-second minimum. The price per second for a running On-Demand Instance is fixed, and is listed on the Amazon EC2 Pricing, On-Demand Pricing page. We recommend that you use On-Demand Instances for applications with short-term, irregular workloads that cannot be interrupted."https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-on-demand-instances.html