The most concentrated solution is the one with the highest molarity. Molarity is calculated by taking the moles of solute/ liters of solution.
The already gave you the molarity, so you do not need to calculate it. The most concentrated solution is the 0.25 M KCl. Keep in mind that the volume they gave you does not effect the molarity of the solution. It just tells us that they have 100 mL of a 0.25 M KCl solution. The 100 mL solution has a molarity of 0.25.
Your correct answer would be D) 100 mL 0.25 M KCl.