Spot Instancesare the most cost-effective EC2 instance purchasing option for batch workloads that can handle interruptions and can be restarted from where they left off. Spot Instances offer significant cost savings (up to 90%) over On-Demand Instances by using spare EC2 capacity. They are ideal for workloads that are fault-tolerant and flexible in terms of timing.

A. Reserved Instances: Incorrect, as they require a long-term commitment and do not offer the flexibility needed for short-term, interruptible workloads.

C. Dedicated Instances: Incorrect, as they are designed to run on hardware dedicated to a single customer, which is more expensive.

D. On-Demand Instances: Incorrect, as they are more expensive than Spot Instances and are used for applications that require immediate or predictable capacity.

AWS Cloud References:

Amazon EC2 Spot Instances