The correct answer is C) If a passing agreement is reached by radiotelephone, whistle signals are optional. The reason this is correct is that under the Inland Navigation Rules, if vessels are meeting on a river and will pass at least 1/4 mile apart, they are not required to exchange whistle signals if they have made a passing agreement via radiotelephone communication. The whistle signals are optional in this scenario, as long as the passing agreement has been reached through verbal communication. The other options are incorrect because A) whistle signals are not required if a passing agreement has been made by radio, B) whistle signals are only required when passing within 1/2 mile, not in all situations, and D) the correct answer is C, not none of the above.