The word that best fills the blank in the sentence is (B.) zeitgeist ("Her poodle skirt and bobby socks caught the zeitgeist of the 50's").
The word 'zeitgeist' comes from German and it is used to refer to the mood or spirit that is typical of a particular period in history or a generation. 'Zeit' means 'time', while 'geist' means 'spirit'. Furthermore, 'zeitgeist' is the correct option because the sentence is referring to a look that was trendy in a specific period in history (the 50's): poodle skirt and bobby socks. This look is what catches the spirit of the 50's.