Option A is correct because this is a sequential, dependent workflow where each validation stage depends on the previous one, and the overall execution can last up to 24 hours. AWS states that Standard Workflows in Step Functions are ideal for long-running, durable, auditable workflows and can run for up to one year. AWS also documents the Wait for a Callback with Task Token pattern, which is specifically designed for workflows that must pause until an external process reports completion. That maps directly to multi-level validations that may complete asynchronously but still need orchestration within a bounded window.

Option B is incorrect because Express Workflows can run for only up to five minutes, so they cannot support a transaction workflow that may take many hours. Option C adds more custom code and operational management than Step Functions and does not naturally provide the same stateful orchestration, dependency tracking, and callback waiting pattern. Option D is also more operationally heavy and is better suited to data pipeline orchestration than per-transaction validation workflows. Therefore, Standard Step Functions provides the lowest operational cost while fully meeting the timing and dependency requirements.