Which of the following instruments is NOT required for IFR flight?

During Instrument Flight Rules (IFR) operations, specific equipment is mandated to ensure safety and compliance with regulations. Among the options listed, a device for communication with air traffic control is essential, but it's not an explicit requirement that can be measured like the other instruments.

A clock with a second pointer is essential for timing, which is critical in maintaining awareness of time-based navigation, particularly when flying under IFR. The radar altimeter provides precise altitude information, particularly in low visibility conditions, although it is not universally required for all IFR flights, making it less universally applicable than the other requirements.

A transponder with altitude reporting is crucial for IFR operations, facilitating the identification of aircraft in air traffic control systems, thus enhancing safety. Therefore, while communication with air traffic is vital, the specific device to achieve this can vary, and it is not explicitly listed as a required instrument. The need for such communication exists but does not necessitate a specific device in the same way as the other instruments do.