Given that the abundances of isotopes  54Fe 56Fe  and  57Fe are 5%, 90% and 5% respectively. The atomicmass of Fe is

Average atomic mass of Fe= (Fractionalabundance of  54Fe× mass of 54Fe+ (Fractional abundanceof  56Fe× mass of 56Fe+ (Fractional abundance of  57Fe×mass of  57Fe. Hence:Atomic mass of Fe=54×5100+56×90100                            +57×5100=2.7+50.4+2.58=55.95So, the correct answer is (b).