"only if a user uploads data to an Amazon S3 bucket" that excludes B & C because we need s3:ObjectCreated:* You don't need SNS for S3 event notifications so A is easier.

A. Create an S3 event notification that has an event type of s3:ObjectCreated:*. Use a filter rule to generate notifications only when the suffix includes .csv. Set the Amazon Resource Name (ARN) of the Lambda function as the destination for the event notification. Explanation: This solution directly triggers the Lambda function only when a .csv file is uploaded to the S3 bucket, minimizing unnecessary invocations of the Lambda function. It uses a specific event type (s3:ObjectCreated:*) and a filter rule to ensure that the Lambda function is invoked only for relevant events. Additionally, it directly invokes the Lambda function without the need for additional services like Amazon SNS, reducing operational overhead.

S3 event notification to lamba for file prefix with.csv is the least overhead way

"You can use Lambda to process event notifications from Amazon Simple Storage Service. Amazon S3 can send an event to a Lambda function when an object is created or deleted" https://docs.aws.amazon.com/lambda/latest/dg/with-s3.html