Answer: B. Prepare the solution again.

Once the liquid level rises above the mark on the volumetric flask, the exact volume of solution is no longer known, so the concentration of the solution cannot be determined with the volumetric flask. Do not remove excess liquid with a medicine dropper; a portion of the solute will probably be removed as well. Adding more solute will not fix the error, since the volume of liquid is still unknown. Do not use the solution as it was prepared. Instead, dispose of the solution in the appropriate waste container and prepare a new solution with the proper volume.