Which of these heterocyclic compounds are aromatic? Explain your answer briefly.

Compounds (a), (c), (d) and (i) are aromatic because they don't have any elements of unsaturation.

Compounds (b), (e), (f) and (k) are aromatic because they all have a planar structure, cyclic π system, and 8 pi electrons.

Compounds (b), (e), (g), (h) and (j) are aromatic because they all have a planar structure, cyclic π system, and 6 pi electrons.

Compounds (b), (e), (f), (g), (h), (j) and (k) are aromatic because they all have a planar structure, cyclic π system, and 6 or 10 pi electrons.