Choice "B" is correct. Adding a job with a positive contribution margin within idle capacity will increase operating income. The company will still make a profit for the special order even though the gross profit percent will be lower. The question states that fixed costs in total will be the same (i.e., the company is operating within the relevant range). Variable costs per unit will be the same. Note: Fixed costs per unit decrease with increased production.Choices "a" and "c" are incorrect. Fixed costs will not increase within the relevant range.
Choice "d" is incorrect. Operating income will increase since the new job has a positive margin and is utilizing otherwise idle capacity.