The correct answer is D) If agreement is reached by radiotelephone, whistle signals are optional. This is because under the Inland Navigation Rules, Rule 34(e) states that "When vessels in sight of one another are approaching each other and from any cause either vessel fails to understand the intentions or actions of the other, or is in doubt whether sufficient action is being taken by the other to avoid collision, the vessel in doubt shall immediately indicate such doubt by giving at least five short and rapid blasts on the whistle." However, the rule also states that "Such signal may be supplemented by a light signal of at least five short and rapid flashes." This means that if the vessels have agreed on a passing arrangement by radiotelephone, the whistle signals become optional, as long as the vessels have reached a clear understanding. The other options are incorrect because A) prohibiting such passing agreements is not consistent with the Inland Rules, B) a vessel that has made a passing agreement is not required to sound whistle signals, and C) whistle signals are not required if a clear passing agreement has been made by radiotelephone.