The correct answer is A) Extend arm with the palm down and hold this position rigidly. This is the correct signaling method to instruct a crane operator to stop, as specified in the Standard Hand Signals for Controlling Crane Operations published by the Occupational Safety and Health Administration (OSHA). Extending the arm with the palm down and holding it rigidly is the recognized standard signal to convey the instruction to stop the crane's movement. The other options are incorrect because they do not match the standard hand signal for "stop" as defined by OSHA regulations. Clasping the hands, using fists with thumbs out, or moving the arms back and forth are not the proper signaling methods for communicating the stop command to a crane operator.