Which of the following is NOT a traffic source (i.e. not a “Default Channel Group”)?

Impressions are a metric measuring how many times an ad or content is displayed, not a source from which users arrive at a website. Traffic sources, or Default Channel Groupings, categorize where users originate. Organic Search refers to visitors from unpaid search engine results. Social includes traffic from social media platforms. Referral traffic comes from links on other websites. Therefore, Impressions is the correct answer because it describes an event (displaying content) rather than a channel bringing users to a site.