Answer
For (a) Indole, you just need to drop the H-atoms and hide the C-atoms. Keep in mind, to draw the N-atom, otherwise you won't know, that an N-atom belongs to this structure.
(b) a classical zig zag line where you just need to drop the H-atoms and hide the C-atoms.
(c) This ring contains halogens which need to be shown. It's because you can't figure out the halogens by just looking at this skeletal structure.
(d) As of this ring, you just remove the C and H atoms but display the O atoms so that you can reconvert the skeletal structure to the original drawing.
Work Step by Step
Regarding rings and carbon chains, carbons and hydrogen atoms aren't explicitly shown. Instead, a carbon chain consists of a zig zag line while for a ring, the carbons are hidden and hydrogens attached to them are dropped.
