All titrant has drained out of the buret, into the reaction flask...

Answer: A) Discard the solution and begin the titration again.

The buret is only calibrated to the 50.00 mL mark, so any volume of liquid past this mark cannot be measured. Since the volume of titrant added to the flask cannot be determined accurately, the solution must be discarded and the titration begun again.

Note: In practice, if more than 50 mL of titrant are needed to reach the endpoint, the solution may have been prepared incorrectly. If indicator is needed for the titration, check to see if indicator was added to the solution.