USCG #4536, Rule Ref 12
BOTH INTERNATIONAL & INLAND
If a sailing vessel with the wind on the port side sees a sailing vessel to windward and cannot tell whether the other vessel has the wind on the port or starboard side, she shall __________________.
(A) hold course and speed
(B) sound the danger signal
(C) keep out of the way of the other vessel
(D) turn to port and come into the wind
This is a question on Rule 12a, part iii. And even though this question is literal wording from the Rule 12a iii itself, it is good for sailors because they must know both the windward-leeward part of the rule (12a-ii) as well as the port-starboard part (12a-i) to answer it, not to mention part 12a-iii itself.
You are on port tack, so a port-tack boat approaching from windward would have to stay clear of you, but a starboard-tack boat approaching from windward would have right of way over you. If risk of collision already exists but you still cannot tell what tack the approaching windward boat is, you must assume it is starboard and "keep out of the way." In short, this is asking a question about a fine detail of the Rule and one, frankly, not likely to come up often these days with modern sail rigs.
For operators of power-driven vessels, this is especially a fine point and one that must be learned to pass a test more than for practical application... although it could be of value in a congested area where the power-driven vessel might have to anticipate the maneuver of two approaching sailing vessels. Sailboats are an anomaly to power driven-vessels in general. The latter are often unable to determine by single observation what tack or direction the sail vessel is on or even which way it is headed. Conversely, sailboats generally are under the notion that they have the right of way and are disinclined or unaware of what to watch for in the interests of their own safety. This question was included to give some exercise in looking at sailboats in the Nav Rules..