Which of the following is NOT a location referencing system used on aircraft?

The correct answer, indicating that "Humidity lines" are not a standard location referencing system used on aircraft, reflects an understanding of aircraft design terminology and measurement systems.

In aviation, location referencing systems like water lines, fuselage stations, and buttock lines are critical for design and maintenance. Water lines are horizontal reference lines established to help in the vertical positioning of components on the aircraft. Fuselage stations provide a way to identify positions along the length of the aircraft, which is particularly useful for alignment and fitting of parts. Buttock lines are used to indicate a position laterally, enabling precision in component placement.

In contrast, humidity lines do not exist as a standard measurement or referencing system in aircraft design or maintenance. The absence of this measurement underscores its inapplicability in aircraft engineering, while the other systems serve vital roles in ensuring structural integrity and proper assembly of aircraft components.