The correct option is
A (a)
1,3-cyclobutadiene is a cyclic, planar conjugated molecule having 4
π-electrons. Therefore, it is not an aromatic compound.
(b)
Pyridine is a cyclic, planar conjugated molecule having 6
π-electrons. Therefore, it is an aromatic compound.
(Here lone pair of electrons of
N are not involved in delocalisation.)
(c)
Furan is a cyclic, planar conjugated molecule having 6
π-electrons. Therefore, it is an aromatic compound.
(d)
Thiophene is a cyclic, planar conjugated molecule having 6
π-electrons. Therefore, it is an aromatic compound.
Hence option (a) is correct.