A DevOps Engineer must improve the monitoring of a Finance team payments microservice that handles transactions for an e-commerce platform. The microservice runs on multiple Amazon EC2 instances. The Finance team would like to know the number of payments per minute, and the team would like to be notified when this metric falls below a specified threshold.
How can this be cost-effectively automated?

A. Have the Development team log successful transactions to an application log. Set up Logstash on each instance, which sends logs to an Amazon ES cluster. Create a Kibana dashboard for the Finance team that graphs the metric.
B. Have the Development team post the number of successful transactions to Amazon CloudWatch as a custom metric. Create a CloudWatch alarm when the threshold is breached, and use Amazon SNS to notify the Finance team.
C. Have the Development team log successful transactions to an application log. On each instance, set up the Amazon CloudWatch Logs agent to send application logs to CloudWatch Logs. Use an EC2 instance to monitor a metric filter, and send notifications to the Finance team.
D. Have the Development team log successful transactions to an application log. Set up the Amazon CloudWatch agent on each instance. Create a CloudWatch alarm when the threshold is breached, and use Amazon SNS to notify the Finance team.