The correct answer is A) Attempt to call the vessel on VHF radiotelephone because someone on board requires medical assistance. The signal flag "W" indicates that the vessel is requesting medical assistance. According to maritime regulations, when a vessel displays the "W" flag, it is required to render assistance and provide medical aid if possible. Proceeding on your current course and speed (options C and D) would not be the appropriate response, as you have a duty to respond to the distress signal. Calling the vessel on the radio (option A) is the correct action to take in order to assess the situation and provide any necessary medical assistance.