The correct answer is C. Adding a trigger to your Lambda function is the solution that will meet the requirement of processing data files immediately after they are uploaded to an Amazon S3 bucket. A trigger is an event that can invoke a Lambda function, and in this case, the trigger must be an Amazon S3 event that fires when a new file is uploaded to the bucket. Options A and B involve different mechanisms that are unnecessary for this scenario, and option D is incorrect as adding a layer does not achieve the intended outcome.