f).
the dotted lines are still there but the big difference is that they have been slightly rounded.
compare this to sketch a to see what i mean. see what happens in sketch f?
even though there are no features on the face, the dotted lines on a curve give the clear impression of a rounded solid object. in this case, it's a head, but it could be a ball, an egg, a cushion or whatever.
now add some basic features following the curved dotted lines as in sketch g and suddenly, with no effort,you have a 3d head. it's that easy!
again, compare sketch g to sketch b. remember, the features must follow the 