In diabetic ketoacidosis, there is increased acid in the extracellular space. According to FA there is a transcellular shift due to decreased insulin that leads to more H+ entering the cell in exchange for K+. This leads to hyperkalemia with depleted intracellular stores of K+. There is also osmotic diuresis that leads to increased K+ loss in the urine and total body K+ depletion. The question asks that is most likely to decrease with insulin therapy: serum potassium concentration will decrease as K+ is now exchanged for H+ inside the cell.