f(x) (에프엑스) is a five member girl group signed with SM Entertainment. The name of their group "f(x)" is a play on the mathematical notation for function, with "f" signifying "flower" and "x" representing the female's double X chromosome. The group includes Victoria (宋茜), Amber (劉逸雲), Luna (박선영), Sulli (최진리), and Krystal (정수정).
The name of this song is Nu ABO (NU 예삐오).
This group also apparently released a version of the song Chocolate Love last year. The song was made popular last year by Girls' Generation's video and you can see by the lopsided hit count for each video which one was obviously more popular.