Your organization has an S3 bucket that stores confidential information. Access is granted to certain programmatic IAM users and restricted these IAM users` requests to be originated from within your organization`s IP address range. However, your organization suspects there might be requests from other IP addresses to S3 buckets to download certain objects. How would you troubleshoot to find out the requester IP address? (choose 2 options)

Answer options:

A.Enable VPC flow logs in the region where S3 bucket exists.
B.Enable server access logging
C.Enable CloudTrail logging using OPTIONS object
D.Enable CloudWatch metrics

Answer: B, C
Server access logging provides detailed records for the requests that are made to a bucket. Server access logs are useful for many applications. For example, access log information can be useful in security and access audits.
For details on how to enable logging for S3, refer to documentation here. 
https://docs.aws.amazon.com/AmazonS3/latest/dev/ServerLogs.html#server-access-logging-overview
For information about the format of the log file, refer to documentation here.
https://docs.aws.amazon.com/AmazonS3/latest/dev/LogFormat.html
For option A, S3 is a managed service and not part of VPC. So enabling VPC flow logs does not report traffic sent to the S3 bucket.
Option B is correct.
Option C is correct. Using the information collected by CloudTrail, you can determine what request was made to Amazon S3, the source IP address from which the request was made, who made the request, when it was made, and so on. This information helps you to track changes made to your AWS resources and to troubleshoot operational issues.
For detailed information about how S3 requests are tracked using CloudTrail, refer to documentation here.
https://docs.aws.amazon.com/AmazonS3/latest/dev/cloudtrail-logging.html#cloudtrail-logging- s3-info
 For option D, although CloudWatch has metrics for S3 requests, this does not provide detailed information about each request. It generates metrics for the number of requests sent for each type.
For more information about S3 CloudWatch request metrics, refer to documentation here. 
https://docs.aws.amazon.com/AmazonS3/latest/dev/cloudwatch-monitoring.html#s3-request-cloudwatch-metrics