Rules of the Road: BOTH INTERNATIONAL & INLAND You see the display of lights shown in illustration...

I don't understand this answer, as an additional white light normally indicates "the length of the tow, measuring from the stern of the towing vessel to the after end of the tow exceeds 200 meters," (Rule 24a). What was the deciding factor to have an answer with length of tow as 150 ft?

Can someone explain why this isn't A?

This one is tricky! When the vessel exceeds 50 m, there must be both a forward and aft masthead light. So, for the answer to be A, there would have to be 4 white lights in the picture - one aft for the size of the vessel, and three forward for the 200 meter tow. By looking at the possible answers and only seeing 50+ meter tugs, you know one white light is an aft masthead light. There are now only two left, indicating a tow under 200 meters.

pg 47 head on view

There is no example of this configuration in the USCG book. The closest is two masthead whites indicating tow and then green and red side lights indicating you are head on. But that's for a vessel under 50 meters. Both possible towing vessels are over 50 meters meaning that you're not seeing 3 whites to indicate a tow longer than 200 meters but rather 2 whites to indicate a tow shorter than 200 meters and 1 white is the other mast's masthead.

I have no idea why anyone would think this had anything to do with a dredge.

- I like this - By looking at the possible answers and only seeing 50+ meter tugs, you know one white light is an aft masthead light. There are now only two left, indicating a tow under 200 meters

@EMILYSAKOWICZ

No, Rule 24(a)(i) replaces the FWD masthead light with 2 or 3 lights having the same characteristic depending on the length of tow (> or < 200m)