Which of the following determines if an aircraft can fly if an instrument is inoperative?

The determination of whether an aircraft can fly with an inoperative instrument hinges on the aircraft's certification for Visual Flight Rules (VFR) conditions. Instruments that are categorized under VFR day type certification are specifically intended for flight during daylight under visual conditions, meaning that certain instruments not essential for VFR operations may be inoperative without jeopardizing the safety or legality of the flight.

In this context, if an instrument is not required by VFR regulations for the type of operation being conducted—assuming visual flight conditions—the aircraft can still legally operate without that instrument in service. This relates to the certification standards set forth for VFR operations, which identify the minimum equipment required for safe operation under visual conditions.

The other choices involve considerations that do not directly tie into the regulatory framework for aircraft operations. For instance, whether an instrument is needed by the pilot or for passenger comfort does not factor into the legal operational status of the aircraft when instruments are found to be inoperative. Similarly, checking instruments from a previous flight does not negate the requirements governed by the aircraft's certification for VFR flight.