To determine the number of moles of KMnO4 that reacted with the sample, we just have to multiply the given volume with the concentration. There is a need to convert the volume to liters first. Below is the solution,
n = (21.05 mL) x (1 L / 1000 mL) x (0.02047 M) = 0.00043 moles
Thus, the answer is letter D.