HELLO,
(NH4)2CO3 is the correct answer
because as it is an ionic compound the decomposition is possible in this compound
(NH4)2CO3 --------------------------> 2NH4+ + CO3^2-
m
hence it will have different average molecular mass
so the moles will be change due to decomposition
so it will show abnormal vapour density = average molecular mass /2
But no other compounds rather than (NH4)2CO3 will able to go decomposition
THANK YOU