The correct answer is D) dry bulb and wet bulb temperatures. A sling psychrometer is a type of hygrometer used to measure the dry bulb and wet bulb temperatures, which are then used to determine the relative humidity of the air. The dry bulb temperature is the actual air temperature, while the wet bulb temperature is the temperature of a water-cooled thermometer, which is lower than the dry bulb temperature due to the evaporative cooling effect. These two measurements allow the calculation of relative humidity, an important factor in maritime operations. The other answer choices are incorrect because a sling psychrometer is not used to measure barometric pressure (A), sea water temperature (B), or engine temperature (C).