S1: A formula is valid iff its complement is not satisfiable
Here, it says a formula is valid(Tautology) iff complement is not satisfiable means unsatisfiable i.e. Contradiction.
So, S1 is true.
S2: A formula is satisfiable iff its complement is not valid
It means, a formula is satisfiable(Tautology or Contingency) iff its complement is not valid or invalid(Contradiction or contingency).
That's also true
So, answer is (c) both S1 and S2.