Which FAR part deals with pilot instrument proficiency?

The regulation that pertains to pilot instrument proficiency is found in Part 61.57. This particular section of the Federal Aviation Regulations addresses the requirements for pilots to maintain their currency and proficiency for flying under instrument flight rules (IFR). Specifically, it outlines the necessary experience and procedures that a pilot must follow to be considered proficient to operate an aircraft solely by reference to instruments. This includes requirements for instrument flight time and the necessity to complete an instrument proficiency check within specific intervals.

The other parts listed do not specifically focus on pilot instrument proficiency. For example, Part 91.205 relates to the basic VFR and IFR equipment requirements for operations, while Part 121 outlines regulations applicable to scheduled air carriers and Part 107 governs the operation of small unmanned aircraft systems (UAS), which is not directly related to pilot instrument proficiency. Thus, Part 61.57 is the correct regulation for understanding the requirements to maintain proficiency in instrument flying.