Answer is C : According to Principle of Duality between Fuzzy Conjunction operator (AND / here t(x,y)) and fuzzy disjunction operator(OR / here s(x,y) ) , they form a dual pair if they satisfy the following condition:
1-t(x,y) = s(1-x, 1-y)
from this condition we'll get t(x,y)=1-s(1-x,1-y)