Which of the following is NOT a required instrument for part 135 VFR operations?

In Part 135 VFR (Visual Flight Rules) operations, the regulatory requirements for the instruments needed in the aircraft are specified to ensure safety and situational awareness for pilots. A Flight Data Recorder is not a required instrument for VFR operations under Part 135. While it may be a valuable tool for collecting data about aircraft performance and operating conditions, it is not mandatory for compliance with the regulations governing VFR operations.

In contrast, instruments like the Altitude Indicator, Turn and Slip Indicator, and Magnetic Compass provide essential information for the pilot during flight. The Altitude Indicator helps pilots maintain their altitude, the Turn and Slip Indicator provides critical information about the aircraft's rate of turn and coordination, and the Magnetic Compass serves as a fundamental navigational aid. Therefore, the Flight Data Recorder stands out as the only option that is not necessary for conducting VFR operations under Part 135.