If a plant is operating at full capacity and receives a one-time opportunity to accept an order at a special price below its usual price, then :

If the Plant is operating at full capacity, it is more likely that the Order will be rejected. 

  I did not understand your question, but I think by Mavhmt answer (c) the nearest

d.the order will likely be rejected 

since the plant is operating at full capacity  

The case new order to be accepted then more space to be rented / built. New machines to be purchased. More utilities expenses to be incurred and more workers are needed. Then both fixed and variable costs are affected. Because selling price would be less than the usual then profit from the offer would be less than the usual. Therefor answer d is more likely.

d. the order will likely be rejected <<<<<<<<<>>>>>>>>>>>>>

d.the order will likely be rejected.

Since the plant is operating at full capacity, an special order (one-time) would mean additional  fixed equipment and thus would mean higher expense, therefore, the answer would be d.the order will likely be rejected.

A- only Variable Costs relevant