A towing vessel 35 meters in length, with a tow 100 meters astern, must show a minimum of how many masthead lights?

The correct answer is based on the requirements established by maritime regulations for vessels engaged in towing operations. A towing vessel that is over 12 meters in length, which in this case it is at 35 meters, must exhibit two masthead lights when it is towing another vessel. This is because the overall length of the tow (the length of the towing vessel plus the length of the tow) exceeds 50 meters, necessitating the two lights to ensure better visibility and identification in navigation.

The two masthead lights serve a critical purpose: they help other vessels determine the type and length of the towing configuration from a distance, thereby reducing the risks of collision. Additionally, the universal standard for lights is designed to ensure consistency in what different vessel types show, promoting safer navigation.

While the other options suggest fewer masthead lights, they do not meet the specific regulatory requirement for towing vessels of this length and configuration. Thus, two masthead lights is the minimum standard to adhere to for effective visibility in towing situations.