Amazon ECS using the Fargate launch type is correct because it bills for the vCPU and memory requested by each running task so costs accrue only while tasks are actually running and you do not manage or pay for idle EC2 instances.

Amazon ECS using the Fargate launch type provides a serverless container model where AWS provisions and meters the compute per task so it fits spiky traffic and the finance requirement to pay only for active tasks.

An Amazon ECS service with Auto Scaling is incorrect because Auto Scaling only adjusts how many tasks run and it does not change whether you are billed per task or per instance so it does not by itself guarantee the per-task billing requirement.

Amazon ECS with the EC2 launch type is incorrect because you are billed for EC2 instances regardless of task activity so you can end up paying for idle capacity when traffic is low.

Amazon ECS with EC2 Spot capacity providers is incorrect because although Spot can lower instance costs you are still billed at the instance level and Spot instances can be interrupted which does not satisfy a strict per-task, always-available billing requirement.

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