Which service does the USPS provide assistance to the Department of State?

The correct answer is related to the service provided in relation to the passport application process. The USPS plays a crucial role in assisting the Department of State by offering passport services, which include accepting passport applications, processing fees, and handling the submission of these documents. This partnership ensures that applicants can conveniently apply for passports at their local post offices, which serves as a reliable and accessible resource for the public.

In contrast, while burial flags are a service provided to honor deceased veterans, they do not involve direct assistance to the Department of State. Registration forms, which could pertain to various applications, may not specifically link to any governmental partnership with USPS in the manner that passport services do. Duck stamps, primarily a means to support wildlife conservation and specific hunting activities, also do not relate to the support of the Department of State. Therefore, the role of USPS in processing passport applications stands out as the most appropriate answer.