In which mode selection does the Mode S transponder report neither Mode C nor Mode S?

The selected answer indicates the scenario where the Mode S transponder is not actively reporting altitude information. In ALT OFF mode, the transponder is set to a state where it does not provide any altitude data, which is essential for differentiating between the various modes of data transmission.

Mode S transponders are designed to report information based on their operational state, which includes altitude data in Mode C, and more detailed information in Mode S. When the transponder is configured to ALT OFF, it effectively disables the transmission of both altitude and Mode S information. This is why ALT OFF is the mode where you can expect the transponder to not report either Mode C or Mode S data, leading to a level of reduced situational awareness for air traffic control.

To contrast this with other modes, NORMAL mode includes standard reporting of altitude and identification, while ALT ON indicates that the altitude reporting is operational, facilitating communication of both Mode C and Mode S data. In STANDBY mode, the transponder is essentially inactive, but it may still have the capability to switch back to reporting modes quickly, depending on the aircraft system design.