Q: What feature is required to collect the number of comments users posted to a web page?
A) Custom Channel Groupings
B) Calculated Metric
C) Custom Channel Groupings
D) Custom Metric
Correct Answer is D) Custom Metric.
custom metrics can produce more flexible and more readable custom reports and as such are a convenient way to track your most important metrics.
Read more here: https://support.google.com/analytics/answer/2709828?hl=en#example-metrics