The correct answer is C) A radiotelephone. According to the Code of Federal Regulations (CFR) for the US Coast Guard, all towing vessels of 26 feet or longer while navigating are required to carry a radiotelephone. This regulation is outlined in 46 CFR 161.004, which states that towing vessels must be equipped with a radiotelephone capable of providing two-way communications. This requirement ensures that towing vessels can effectively communicate with other vessels and shore-based facilities for safety and coordination purposes. The other answer choices are incorrect. A radio direction finder (option A) is not a mandatory requirement for towing vessels of this size. Carrying at least two lifeboats (option B) is not a general requirement for towing vessels. Option D is also incorrect, as the regulation does mandate the use of a radiotelephone on towing vessels of 26 feet or longer.