All of the following are examples of active leisure activities except:
a. playing chess
b. kickboxing
c. sketching a portrait
d. listening to music

D listening to music.

This is because one is mobile in every other option; in chess, moving your arm to move the chess piece, in kickboxing you are kicking, when you sketch a portrait, you move your arm but when you listen to music, you don't necessarily move, thus not being active! :D 

The one that most represents the earths mantle would be the skin since the mantle is the upper part of the earths depths which is just like the skin of an apple.