A ‘Source’ refers to the actual source from where the visitor was referred to your site. Among the options, ‘googlemerchandisestore.com’ is a source, ‘Email’ is not a source, it is a Medium. ‘Google’ is a source and ‘(direct)’ is also a Source.
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.