Final answer:
Amazon SWF (Simple Workflow Service) is the correct AWS service for coordinating tasks across distributed application components, useful for scenarios such as order fulfilment workflows.
Step-by-step explanation:
The AWS service that can assist with coordinating tasks across distributed application components in a business process workflow, such as an order fulfilment system, is Amazon Simple Workflow Service (Amazon SWF). Amazon SWF helps developers build, run, and scale background jobs that have parallel or sequential steps. You can think of Amazon SWF as a fully-managed state tracker and task coordinator in the Cloud.
If we look at the other services mentioned, Amazon Simple Queue Service (Amazon SQS) is used for message queuing for service-to-service communication, Amazon Simple Notification Service (Amazon SNS) for pub/sub messaging, and Amazon Security Token Service (Amazon STS) for temporary, limited-privilege credentials for AWS resources. For the specific needs of a workflow of distributed tasks, Amazon SWF is the most appropriate choice.