According to Huckel’s rule for aromaticity, planar monocyclic rings containing (4n+2)n electrons (where n=0,1,2,3 and so on) should be aromatic. Another requirement for aromaticity is planarity of the ring. In the given compounds, all contains 6n electrons, therefore for them, n comes out be 1,hence, pyrrole, thiophene and furan are all aromatic.
