A company wants to migrate its web application to AWS and leverage auto scaling to handle peak workloads.
The solutions architect determined that the best metric for an auto scaling event is the number of concurrent users.
Based on this information, what should the developer use to auto scale based on concurrent users?

A. An Amazon SNS topic to be invoked when a concurrent user threshold is met
B. An Amazon Cloudwatch NetworkIn metric
C. Amazon CloudFront to leverage AWS edge locations
D. A custom Amazon CloudWatch metric for concurrent users