Answer to the question no:1
(1)Answer: Sunk cost has no resale value that's why it's not considered as relevant cost.
Answer to the question no:2
(2)Answer: Opportunity cost means when one alternative is chosen and reject other alternatives and the cost of 2nd best alternative is called Opportunity cost.So in decision making when a person reject a alternative also reject the profit or income of this alternative so he lost a alternative.So this Opportunity also carry a cost for taking first alternative.
Answer to the question no:3
(3)Answer: Direct Material,Direct Labour,variable manufacturing overhead,fixed manufacturing overhead traceable and those has resale value theses are relevant cost in times of buy or make decision.
Answer to the question no:4
(4)Answer: Traceable fixed cost has resale value and in particular it's traceable also this cost is actually happen for a particular product that's why this cost is considered as relevant cost.