Final answer:
The complete subject of the sentence is 'Popular music, films, books, and fashions' and the simple subject is 'music'.
Step-by-step explanation:
The complete subject of the sentence 'Popular music, films, books, and fashions are the sources of our common culture.' is 'Popular music, films, books, and fashions', as it includes all of the nouns and their modifiers that tell us what or who the sentence is about. The simple subject, on the other hand, is 'music', which is the main noun of the complete subject that the sentence is focused on.