Which statement about GACA publications is true?

The statement that GACA Technical Standard Orders are all approved data is indeed true because Technical Standard Orders (TSOs) issued by GACA provide the standards that certain products must meet to be considered airworthy. These documents represent approved criteria that manufacturers must follow when producing aviation products and parts. If a manufacturer complies with the applicable TSO, their products are deemed to meet specific safety and performance criteria set by GACA.

In the context of this question, the other statements do not align with the established regulations or practices. For example, implying that only manufacturer's data can be approved overlooks the broader scope of regulatory approval processes which include TSOs and other certifications. Furthermore, while Airworthiness Directives are critical compliance documents, they are not strictly required to be part of the GACA publications themselves. Thus, the focus on GACA Technical Standard Orders as a form of approved data helps clarify the integrity of the materials related to aviation safety and regulation.