The correct answer is C) The actual firing is at the discretion of the Master. The requirement to test the line throwing appliance on a vessel does not mandate the use of a regular service line when it is fired. The regulations allow the Master to decide whether to actually fire the device or conduct a dry run during the required quarterly drill. As long as the drill is performed, the Master can choose to either fire the device with the regular projectile or simply go through the motions without live firing. The other options, such as using a regular service line (A) or conducting the drill every three months (D), are not specific requirements for testing the line throwing appliance.