In redox reactions Equivalent weight of oxidant or reductant is determined based on the no of electrons gained or lost by them during the reaction.
Thus, E = Molecular Weight/(Number of electrons lost/gained)
In the reaction FeS2 --> Fe2O3 + SO2,
Fe changes its oxidation state from +2 to +3. Thus it looses one electron. While both the Sulpher atoms(One being in 0 Oxidation State while the other being in -2 Oxidation state) change to +4 Oxidation state. Thus, in total they loose 10 electrons. Thus, the correct answer is M/11