Choice "B" is correct. The opportunity cost is the opportunity foregone, the $150,000 in lost contribution from not taking the new job:

Choice "c" is incorrect. The opportunity foregone is not the amount of the gross sales foregone.Choice "a" is incorrect. The opportunity foregone is not the difference between sales under either alternative ($600,000 - [$1,600,000 x 25%]) or $600,000 - $400,000$200,000.Choice "d" is incorrect. The opportunity foregone is the lost contribution of the alternative, NOT the difference between the contribution margin of the proposed alternative ($150,000) and the current 15% contribution margin of $60,000 ([$1,600,000 x 25%] x 15%). The difference of $90,000 ($150,000 - $60,000) is the differential cost (and highly relevant to decision-making), but it is not the opportunity cost.