An aromatic compound can follow Huckel's rule if it has (4n+2)π -electrons where, n= integer 0,1,2,3,....... etc. Thus, (a) n=0;4×0+2=2π∴ follow Huckel's rule (b) n=1; Number of π -electrons =6 ∴ follows Huckels rules. (c) n=1; Number of pi -electrons =6 ∴ follows Huckel's rule. (d) n=1, Number of pi -electrons =8 (4n+2=4×1+2=6) but this compound has 8 electrons which distorts the plane and hence does not follows Huckel's rule.