Which of the following requires a custom CloudWatch metric to monitor?

A.
Data transfer of an EC2 instance

B.
Disk usage activity of an EC2 instance

C.
Memory Utilization of an EC2 instance

D.
CPU Utilization of an EC2mstance

Explanation:
http://docs.aws.amazon.com/AmazonCloudWatch/latest/DeveloperGuide/ec2-metricscollected.html
CPU, Disk I/O, Data Transfer are default metrics. Memory is not mentioned.