Custom Metric is required to collect the number of comments users posted to a web page.
- Custom Dimension
- Calculated Metric
- Custom Channel Groupings
- Custom Metric
Correct answer is: Custom Metric
Explanation: 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