structural isomers

[ppkjref]

How many structural isomers are possible for 1 atom of hydrogen, 1 atom of oxygen, carbon, and 1 atom of nitrogen?

a) 2
b) 3
c) 4
d) 5
e) 6

HCNO
HCON
HNOC
HNCO
HOCN
HONC

These are the only possible structural isomers (unless one does not work)