A Medium refers to the actual medium from where the visitor was referred to your website. From the options above, Referral is a medium, Google is not a medium, it is a Source. CPC is also a medium that indicates Paid Search advertising and Organic is also a medium that indicates Organic Search traffic.
This question is a part of the Google Analytics Individual Qualification. You can get all the answers with detailed explanation for this exam in our Google Analytics Individual Qualification Exam Answers page.