She is responding to a statement he made, and this is the reply to that, so it's Bingley. Besides, I cannot see Darcy calling her "my dear Caroline"! ;-)

"If you mean Darcy," cried her brother, "he may go to bed, if he chuses, before it begins ...."

"I should like balls infinitely better," she replied, "if they were carried on in a different manner...."

"Much more rational, my dear Caroline, I dare say, but it would not be near so much like a ball."