Which statement is true regarding the use of approved data for major alterations?

The correct answer highlights an important aspect of aviation regulations regarding major alterations. When modifications are made to an airframe or component, it's crucial to ensure that the data used for these modifications remains valid. If there have been prior alterations that conflict with the new data or proposed modifications, then the data can indeed become invalid. This could compromise the overall safety and compliance of the aircraft because any conflict may result in reduced performance or even unsafe conditions.

By recognizing this factor, operators and maintenance personnel must carefully evaluate all approved data in the context of the aircraft's history and current configuration. This ensures that all applied data is conducive to safe operation and in compliance with regulatory standards.

The other choices present misunderstandings about approved data. For instance, stating that a manufacturer's manual is not approved data is incorrect, as these manuals are often accepted as standard and approved methodologies for maintaining and modifying aircraft. Suggesting that a Supplemental Type Certificate (STC) must always be used is misleading; while STCs are crucial for certain modifications, not all alterations require one. Lastly, saying that any data can be used without verification contradicts the essential practice in aviation maintenance, which mandates that any modification data must be validated and approved to ensure safety and compliance.