To understand how many users are coming from various devices, like computers or mobile phones, you run a report that shows this data, per device, over the past 30 days. In this report, which of these is considered a "metric" in Google Analytics?

How many users were on mobile phones is correct because in Google Analytics a metric measures quantitative data about user interactions, and counting the number of users on mobile phones represents a numerical value that can be aggregated and analyzed. In contrast, device types are dimensions that describe the category of data, referral sources indicate qualitative attributes, and the date range is a report setting rather than a measure of user behavior.