To identify which system has failed when a system transmitting information to the instruments has malfunctioned, which instruments should be referred to?

Referring to the magnetic compass and vacuum pressure gauge is the correct approach to identify which system has failed when a system transmitting information to the instruments malfunctions. The magnetic compass operates independently of electrical systems, relying instead on the Earth's magnetic field for orientation. Therefore, it can provide a reliable reference even if other systems fail.

The vacuum pressure gauge is critical for indicating the functionality of the vacuum system, which powers instruments such as the attitude indicator and heading indicator. When the vacuum system is operational, the instruments relying on it will function correctly. Observing the magnetic compass allows pilots to cross-check directional information while the vacuum pressure gauge can confirm whether the vacuum system is functioning, offering insights into possible failures.

In contrast, the other options rely more on interconnected systems that may be affected by a single failure, which can make it difficult to isolate the problem accurately. Instruments such as the attitude indicator and altitude indicator depend on electrical inputs, while the vertical speed and airspeed indicators may similarly be influenced by atmospheric pressure and other system-related factors. The heading indicator and turn coordinator also rely on systems that could fail simultaneously, complicating diagnosis. Thus, focusing on the independent magnetic compass and the vacuum pressure gauge provides clearer insights into which specific system has encountered a malfunction.