Placards required on an aircraft are specified in:

The correct choice reflects that the requirements for placards on an aircraft are outlined in the Aircraft Specifications or Type Certificate Data Sheet (TCDS). The TCDS is a critical document that provides essential information about the aircraft's design, operational limitations, and equipment, including the necessary placards that must be displayed.

Placards serve various purposes such as providing safety information, operational restrictions, and guidance to both pilots and maintenance personnel. The specifics regarding which placards are needed for a particular aircraft model can be found in the TCDS, making it an authoritative source for ensuring compliance with regulatory standards and maintaining safe operations.

While maintenance manuals, pilot guides, and service manuals may contain useful information and guidance related to aircraft operation, safety, and maintenance procedures, they do not typically detail the required placards in the same regulatory context as the TCDS. Thus, referring to the TCDS is essential for verifying the exact requirements regarding placards on an aircraft.