When should a vessel agree to be overtaken according to signal rules?

Answer: D — Upon receiving the overtaking signal

A vessel should agree to be overtaken upon receiving the overtaking signal. This is aligned with COLREGs Rule 34, which specifies that when a vessel desires to overtake another, it must indicate its intention with a specific signal. This signal is critical as it establishes the overtaking maneuver's intent and allows the vessel being overtaken to take necessary actions to facilitate safe navigation. When the overtaking signal is received, it indicates that the overtaking vessel is ready to carry out the maneuver, and the vessel being overtaken should respond appropriately, ensuring that there is clarity between both parties. This agreement is essential for maintaining safety at sea, as it helps prevent misunderstandings and potential collisions. The other options involve scenarios where the context for overtaking is not appropriate or relevant to the signaling rules. For instance, being anchored or in restricted waters may limit movement or create conditions unsuitable for safe overtaking. Similarly, the location does not dictate the need for an overtaking signal, making it crucial that the response to the signal governs the actions of the vessels involved.