Final answer:
The standard range for the visibility of a masthead light on a vessel over 50 meters in length is 6 nautical miles, with shorter visibility ranges applying to smaller vessels, which is not represented by the options provided in the question.
Step-by-step explanation:
The range of visibility for a masthead light on a vessel is contingent upon the size of the vessel and is stipulated by international regulations known as the International Regulations for Preventing Collisions at Sea (COLREGs). For most vessels, a masthead light should be visible at a minimum range of 6 nautical miles for a vessel of 50 meters or more in length. For smaller vessels, this range may be reduced. The option of 5 nautical miles, which is indicated as 'b', is not a standard visibility range for masthead lights, thus making this a trick question. The correct answer should align with common navigational standards which recommend a minimum range of visibility that is typically not expressed in any of the options provided.