Final answer:
A Tier 1 ISP connects with National ISPs, provides services to their customers, and sells access to local ISPs, being at the top of the Internet hierarchy.
Step-by-step explanation:
The student has inquired about the type of Internet Service Provider (ISP) that connects with National ISPs, provides services to their customers, and also sells access to local ISPs. The correct answer to this question is A) Tier 1 ISP. Tier 1 ISPs are large national or international ISPs that own and manage their own networks. They are at the top of the Internet hierarchy and exchange traffic with other Tier 1 ISPs on a peering basis, without having to pay for transit through other networks. In contrast, Regional ISPs (Option B) typically operate within a specific geographic area, Local ISPs (Option C) provide services mainly to end users or local businesses in smaller areas, and National ISPs (Option D) operate on a country-wide basis but may rely on Tier 1 ISPs for backbone access and connectivity.