The correct answer is C) must be exchanged if passing agreements have not been made by radio. The Inland Navigation Rules state that when two power-driven vessels are meeting on a clear day and will pass less than half a mile apart, they must exchange whistle signals if they have not made passing agreements by radio. This is to ensure clear communication and coordination between the vessels to avoid a collision. The other options are incorrect because A) exchanging whistle signals is not mandatory in all situations, and B) and D) do not accurately reflect the Inland Navigation Rules requirement for whistle signal exchange when no radio agreement has been made.