The correct answer is B) two masthead lights in a vertical line instead of either the forward or after masthead lights. This is correct because the International and Inland Navigation Rules require a power-driven vessel towing another vessel astern, where the length of the tow is less than 200 meters, to display two masthead lights in a vertical line instead of the forward or after masthead lights normally required. This unique light configuration helps identify the vessel as one engaged in towing. The other options are incorrect because they do not match the specific lighting requirements for a power-driven vessel towing another vessel with a tow length less than 200 meters. Option A is incorrect as it calls for three masthead lights, while options C and D describe different light configurations not applicable to this scenario.