The correct answer is D) estimated position. An estimated position on a nautical chart is typically enclosed by a square to distinguish it from a fix, which is a known position determined by taking bearings or other position-finding methods. An estimated position is calculated based on the vessel's speed, course, and time, and represents the navigator's best guess as to the vessel's location. This is in contrast to a fix, which is a more accurate position determined through direct observation. The other options are incorrect because a running fix is a series of estimated positions used to plot a vessel's course, and a dead reckoning position is a calculated position based solely on the vessel's speed, course, and time, without considering other factors.