Which instrument is primarily used to determine an aircraft's attitude during instrument flying?

The instrument that is primarily used to determine an aircraft's attitude during instrument flying is the attitude indicator. This instrument displays the aircraft's orientation relative to the horizon, showing whether the wings are level or if the aircraft is climbing, descending, or turning. It provides crucial information necessary for maintaining controlled flight under instrument conditions, where visual references outside the cockpit are often not available.

The other instruments mentioned do play important roles in flight, but they do not provide the specific information about the aircraft's attitude. The vertical speed indicator shows the rate of climb or descent, the altimeter provides altitude data, and the speed indicator displays the aircraft's airspeed. None of these instruments indicate the aircraft's spatial orientation, which is essential for safe flying, particularly in challenging weather conditions or when the pilot is relying solely on instruments.