Conditional proof: [(P • Q) • R] ⊃ S / P ⊃ [Q ⊃ (R ⊃ S)]

a) P ⊃ [Q ⊃ (R ⊃ S)]
b) P ⊃ [Q ⊃ (R ⊃ S)]
c) [(P ⊃ (Q ⊃ R)) ⊃ S]
d) [(P ⊃ (Q ⊃ S)) ⊃ R]