BOTH INTERNATIONAL & INLAND You do NOT understand the course or intention of an approaching vessel. Which signal should you sound?

Correct Answer: Option D — Sound not less than five short blasts

Rule 34(d) dictates that when vessels in sight of one another are in doubt as to the intentions or actions of the other, the vessel in doubt shall indicate this by sounding at least five short and rapid blasts on the whistle. This is the standard danger signal for such situations.