Option C is correct.
Tautology means a Valid formula. In digital logic, Any Valid(Always true) boolean expression can thought of as a Tautology.
In Logic, A propositional logic formula is called Valid/Tautology when it is True under all the possible assignments of truth values(T/F) to its Variables.
for eg : $A + A' = 1$ is a Valid boolean formula as we know. Hence, It is a Valid/Tautology.
And We know, in digital logic, if a statement is true, then its dual statement is also true and this dual statement is obtained by changing $+$ for $.$, and $0$ for $1$ and vice versa.
So, If we take Dual of any Tautology/Valid formula in digital logic then We get another Valid/Tautology formula.