The correct answer is D) mean high water. The reference datum used in determining the heights of land features on most nautical charts is the mean high water level. This is the average of all the high tide levels observed over a long period of time, typically 19 years. This datum is used because it provides a consistent reference point for charting the elevation of coastal features, which is important for safe navigation. The other options are incorrect because they represent different tidal datums that are not typically used for determining the heights of land features on charts. Mean sea level (A) and half-tide level (C) are not directly related to the high tide, while mean low water (B) represents the average of the low tide levels, which would not be an appropriate reference for charting land elevations.