Final answer:
A National ISP is the type of ISP that provides services to customers and sells access to regional and local ISPs.
Step-by-step explanation:
The type of ISP that provides services to their customers and sells access to regional and local ISPs is a National ISP. National ISPs are large-scale internet service providers that have a wide network coverage and infrastructure. They typically offer services to both residential and business customers and have the capability to sell access to regional and local ISPs.