Design a minimum two-level NOR realization for the logic function ( f(a, b, c, d) = ∑ (1,2,3,5,6,7,8,9,12,14) ).

a) ( overlinea + overlineb + overlinec + overlined )
b) ( (overlinea + overlineb + overlinec) ⋅ (overlinea + overlined) )
c) ( (overlinea + overlineb) ⋅ (overlinec + overlined) )
d) ( (overlinea + overlineb) ⋅ (overlinec + overlined) + (overlinea + overlined) )