Which delivery service claims to reach every address in the nation?

The United States Postal Service (USPS) claims to reach every address in the nation, which is a critical aspect of its mission. Unlike private carriers, USPS has a unique obligation to provide mail delivery to all residential and business addresses across the country, including remote and rural areas. This universal service requirement means that no matter where someone lives in the United States, they can rely on USPS to deliver their mail.

Other delivery services like FedEx, UPS, and DHL operate on different business models and often focus on specific regions or types of deliveries. They may not guarantee delivery to every single address, especially in more remote areas where it may not be feasible or cost-effective. Therefore, the USPS stands out as the only service that promises delivery to every address, making it the correct answer in this context.