Which piece of equipment is NOT required for IFR under 14 CFR 91.205(d)?

The choice stating "Magnetic compass only" is correct because, under 14 CFR 91.205(d), the regulations specify that while aircraft must be equipped with certain instruments for IFR operations, a magnetic compass is not listed as a required piece of equipment for IFR flight when it is the only source of heading information. The regulations focus on more essential equipment such as two-way radio communication, navigation instruments, and altimeter systems that ensure safe and effective operation in instrument flight rules.

It's important to understand that while a magnetic compass is a useful reference and may be required in VFR (Visual Flight Rules) flights, it is not mandatory in IFR conditions when other more reliable navigation and heading instruments are available. The gyro horizon indicator and sensitive altimeter are specified and critical for flying in instrument conditions since they provide vital information on aircraft attitude and altitude, both of which contribute to maintaining safe IFR operation.