Determine the wind and temperature aloft forecast for DEN at 9,000 feet.

The correct answer provides valuable information about the wind and temperature aloft conditions specific to Denver (DEN) at an altitude of 9,000 feet.

In this case, the wind is indicated as 230° true at 21 knots, and the temperature is -4 °C. Understanding wind direction in terms of true north rather than magnetic north is crucial in aviation, as it standardizes navigation and flight planning by referencing the Earth's geographic poles. The temperature reading is crucial for flight operations, particularly relating to aircraft performance, since it affects engine efficiency, lift, and overall aircraft handling characteristics. A temperature of -4 °C indicates that the air is relatively colder at higher altitudes, which is typical in the atmosphere.

Knowing the specifics of winds aloft, such as speed and direction, helps pilots understand potential flight conditions, including turbulence or drift, and enables them to make informed decisions regarding altitude changes during their flight.

The other options either present incorrect wind speeds, incorrect temperature readings, or wrong references (magnetic instead of true), making them unsuitable for planning and operating a flight effectively.