What part of the FAR regulations pertains to general operating guidelines?

FAR 91 pertains to general operating guidelines that govern the operation of aircraft, primarily in general aviation contexts. This section covers a wide range of regulations, including pilot responsibilities, aircraft maintenance, and general flying rules. It establishes the baseline for safe flying practices and procedures applicable to various types of operations, not just commercial or air carrier operations.

In contrast, the other options focus on specific categories of flight operations. FAR 135 relates to on-demand (charter) operations and supplemental operations, while FAR 121 outlines regulations for scheduled air carriers, including requirements for operating certificates and operational specifications. FAR 141 is specific to pilot schools and addresses the regulations governing the conduct of flight training under a structured curriculum. Hence, FAR 91 serves as the foundational regulation for general operations, making it the correct choice for this question.