The Markownikoff's rule is best applicable to the reaction C3H6+ HBr i.e. unsymmetrical alkene with HBr. This rule is used as a guidance for electrophilic additions in those cases where an asymmetrical reagent is added to an asymmetrical alkene. Where the negative part of reagent attaches itself to that carbon atom that contains lesser number of hydrogen atoms. In options (b) and (d), reagent used is Br2 and Cl2 that given addition reaction via free radial substitution. In option (a), alkene used is symmetrical. Hence, Markownikoff's rule is not applicable there.