The correct answer is B) using a soft wooden plug. In an emergency situation where there is a small hole (not over three inches in diameter) in the hull below the waterline, a soft wooden plug is the appropriate temporary measure to seal the hole. This is because a wooden plug can be easily and quickly inserted into the hole to prevent further flooding, which is the primary concern in this type of emergency. Other options, such as welding a plate patch or rigging a collision mat, may not be feasible or effective in a time-sensitive situation. The other answer choices are incorrect because they either require specialized equipment or techniques that may not be readily available in an emergency (A and D), or they would not effectively seal the hole (C).