In your example, the correct answer is ''i.e.''
''Namely'' and ''i.e.'' are both used to give more information about something that you have just mentioned.
1) 'namely' = you use 'namely' to say exactly what you mean when you have just referred to something in a general or indirect way. It means ''specifically.''
E.g.: ''One group of people seems to be forgotten, namely pensioners.''
E.g.: ''We shall answer/atone for our misdeeds, namely our sins.''
2) 'i.e.' = you use 'i.e.' when you are giving an explanation of a word or expression that you have just used. It has the meaning of ''in other words'', ''that is (to say).''
E.g.: ''You must be an amateur, i.e. someone who has never competed for prize money in athletics.
E.g.: ''Are you familiar with the popular social media? i.e., Facebook, Instagram, Skype, Whatsapp and so forth.''
Hope that helps!!!