The correct answer is B) It measures apparent wind speed. An anemometer on a moving vessel measures the apparent wind speed, which is the combination of the true wind speed and the vessel's speed and direction. This is because the anemometer is measuring the wind relative to the vessel's movement, not the true wind experienced at that location. The true wind speed and direction can be calculated from the apparent wind speed and direction, as well as the vessel's speed and course, but the anemometer itself does not directly measure the true wind. The other options are incorrect because A) an anemometer does not measure true wind speed on a moving vessel, C) it only measures apparent wind speed and not true wind direction, and D) it does not measure true wind speed or true wind direction directly.