A company has multiple branch offices that each upload a daily sales report to a shared Amazon S3 bucket at a set time. A single AWS Lambda function processes all branch reports in one run and writes results to a database. The processing must start once per day at a specific time. Which option meets these requirements most cost-effectively?

An Amazon EventBridge scheduled rule is the most cost-effective and appropriate solution. EventBridge (formerly CloudWatch Events) allows you to define a schedule (using cron or rate expressions) to invoke a target, such as a Lambda function, at specific times. This directly meets the requirement for the processing to start once per day at a set time. S3 event notifications would trigger the Lambda function multiple times, once for each report upload, which is not desired as processing should occur once daily. Running the Lambda function continuously is inefficient and costly, as Lambda is designed for event-driven, short-lived executions, not continuous operation. AWS Step Functions could work but is overkill for a simple scheduled invocation and would incur additional costs for state transitions compared to a direct EventBridge rule.