You create a report to show the types of devices users have been coming from over the past 30 days — whether they were on computers or mobile phones, for example. In this report, which of these is a "metric" in Google Analytics?

In Google Analytics, a metric is a quantitative measurement, something that can be counted or summed. "How many users were on mobile phones" is a count of users, making it a metric. "The device types, desktops and mobile phones" are dimensions, which describe characteristics of your users, their sessions, or actions. The "date range setting for the last 30 days" is a filter or configuration for the report, not a metric or dimension itself. "Where users came from before reaching your site" describes the source of traffic, which is a dimension (e.g., "Source" or "Medium").