A Medium refers to the actual medium from where the visitor was referred to your website. From the options, ‘Organic’ is a medium, ‘CPC’ is a medium that represents paid advertising, ‘referral’ also represents Medium.
‘mail.google.com’ represents the Source and not the Medium.
This question is a part of the Assessment section of the Google Analytics For Beginners Certification course. You can find the answers with full explanation for all the assessment questions on our Google Analytics For Beginners Assessment 3 Answers page, and Google Analytics for Beginners Assessment Answers page.