Which 'mediums' can be used in Google Analytics?

The correct choice among the given options is 'organic.' In Google Analytics, the term "medium" refers to a specific channel through which traffic comes to the site. It denotes the general category of the source of the traffic. For example, "organic" indicates traffic that comes from unpaid search results, typically generated by search engines.

Using "organic" as a medium is essential in understanding how users find a website through search engines without having to pay for the placement of the link. This information helps businesses and marketers assess the effectiveness of their search engine optimization (SEO) efforts.

The other options do not accurately represent valid mediums in the context of Google Analytics. "Cpc" refers to a specific type of traffic (cost-per-click) associated with paid search advertising rather than a broad medium itself. "Mail.google.com" is a specific domain related to Gmail, which doesn’t qualify as a medium but rather represents a source of traffic. Finally, "referral" does denote a way users might arrive at a site, but it is not as specific as "organic" in categorizing traffic channels. Thus, recognizing "organic" as a medium is vital for understanding traffic sources within Google Analytics.