What is a critical function of the "121 rules" for commercial pilots?

The "121 rules" refer to the set of regulations outlined in Part 121 of the Federal Aviation Regulations (FARs), which governs commercial air transport operations. A critical function of these rules is to establish crew requirements and operational procedures for scheduled air carriers. This includes specifications for minimum flight crew qualifications, rest requirements, and operational control, which enhance safety and efficiency in commercial aviation.

These rules ensure that airlines operate under standardized regulations, contributing to a predictable and safe operation for both crew and passengers. By delineating these requirements, the "121 rules" help to create a framework that supports safe and efficient operations in the complex environment of commercial aviation. This systematic approach also aids regulatory oversight and ensures that air carriers meet strict safety standards.

The other options pertain to areas outside the primary focus of Part 121 regulations and do not encompass the comprehensive regulatory measures applicable to scheduled air transport operations. Therefore, option B accurately captures the essence of a critical function of the "121 rules."