Final answer:
The certification information for aircraft under 12,500 lbs can be found in 14 CFR Part 23 of the FARs.
Step-by-step explanation:
The certification of an aircraft under a gross weight of 12,500 lbs can be found in 14 CFR Part 23 of the Federal Aviation Regulations (FARs).
Part 23 specifically deals with Airworthiness Standards: Normal, Utility, Acrobatic, and Commuter Category Airplanes.
This part of the FARs contains the requirements and regulations for certifying small aircraft, including those with a gross weight of 12,500 lbs or less.