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A is corrent. Relevant costs are those future costs that differ between the alternatives under consideration. The direct material (DM) and direct manufacturing labor (DML) are both relevant as these costs would not be incurred if a buy decision were made. Of the $20 of factory overhead, 60% is irrelevant because it is not affected by make or buy decisions. In other words, $12 (.60 x $20) would not change regardless of the make or buy decision. Therefore, total relevant manufacturing cost is $15 ($2 DM + $5 DML + $8 overhead avoided if the buy decision is made).
B is incorrect. Relevant costs are those future costs that differ between the alternatives under consideration. 
C is incorrect. Relevant costs are those future costs that differ between the alternatives under consideration. 
D is incorrect. Relevant costs are those future costs that differ between the alternatives under consideration.