The correct answer is B) used solely for assistance towing. According to the regulations, towing vessels that are used solely for assistance towing, such as for docking or undocking operations, are not required to be fitted with radar. This is because these vessels typically operate in restricted waters where the need for radar is less critical compared to vessels engaged in long-distance towing or offshore operations. The other options are incorrect because: A) pushing ahead does not exempt a towing vessel from the radar requirement, C) the vessel's length is not the determining factor, and D) the vessel's area of operation on the Western Rivers does not automatically exempt it from the radar requirement.