What is the primary skill tested during an IFR checkride?

The primary skill tested during an IFR checkride is the ability to navigate and operate under instrument conditions. This is because flying in Instrument Flight Rules (IFR) requires pilots to rely solely on instruments for navigation and control of the aircraft, especially when visibility is poor or when flying in clouds. The checkride is designed to ensure that a pilot can interpret and utilize the information provided by various instruments, manage the aircraft safely in controlled airspace, and execute procedures such as approaches, departures, and emergency protocols effectively while under IFR conditions.

In this context, the other choices focus on skills that are less relevant to an IFR environment. For instance, operating under visual conditions and visual flight maneuvers do not apply when a pilot is required to fly solely by reference to instruments. Knowledge of aircraft systems, while important, is more about understanding the aircraft's physical capabilities and less about the proficiency needed for instrument navigation and control. Thus, the emphasis of an IFR checkride is firmly on the ability to successfully navigate and operate in situations where visual references are absent, making the selected answer the most appropriate.