Which service automates the detection of potentially fraudulent activities online?

Amazon Fraud Detector is designed specifically to identify and flag potentially fraudulent activities in a variety of online transactions. This service employs machine learning models that are trained on historical transaction data to help businesses detect anomalies and recognize patterns indicative of fraud. By leveraging AWS's machine learning capabilities, Amazon Fraud Detector allows users to create, train, and deploy fraud detection models with ease, making it a powerful tool for enhancing online security and trust.

The other options do not directly serve the purpose of fraud detection. Amazon S3 is a scalable storage service that facilitates data storage and retrieval. Amazon Rekognition is focused on image and video analysis, including object and facial recognition, rather than fraud analysis. Finally, Amazon Textract is designed for extracting text and data from scanned documents and images, which is unrelated to identifying fraudulent activities. Thus, the specific functionality of Amazon Fraud Detector in automating fraud detection makes it the correct choice.