Which application type must disclose its subject matter in the parent application?

The correct choice is the divisional application. A divisional application must disclose the same subject matter as the parent application because it is derived from it. The primary purpose of a divisional application is to allow the applicant to separate claims that the United States Patent and Trademark Office (USPTO) has identified as being directed to more than one invention from the parent application.

By maintaining the same subject matter, the divisional application ensures that the claims are properly supported by the disclosure in the parent application. This requirement upholds the principle of patent law that prohibits new matter from being introduced in patent applications, ensuring that the scope of the invention remains consistent with what was originally filed.

The other application types have different requirements: a provisional application does not require any claims and serves to establish an early filing date; a continuation application can add new claims but does not have to strictly limit itself to the subject matter of the parent, as long as it relates to the same invention; and a continuation-in-part application can include new matter not disclosed in the parent application. Therefore, the requirement for the divisional application to disclose its subject matter in the parent application distinguishes it from the others.