Final answer:
The two types of Integrated Services Digital Network (ISDN) circuits are Basic Rate Interface (BRI) and Primary Rate Interface (PRI), designed for small scale and large scale use respectively.
Step-by-step explanation:
The two types of Integrated Services Digital Network (ISDN) circuits are a) BRI (Basic Rate Interface) and b) PRI (Primary Rate Interface).
BRI is designed for home and small enterprise use, typically providing two 64 Kbps 'B' channels for voice or data, and one 16 Kbps 'D' channel for signaling and control. PRI, on the other hand, is intended for larger organizations, offering 23 'B' channels and one 'D' channel in North America and Japan (1.544 Mbps, known as a T1), and 30 'B' channels and one 'D' channel in Europe (2.048 Mbps, known as an E1).
It's important to note that c) DSL (Digital Subscriber Line) and d) T1 are types of high-speed Internet connections and not specific to ISDN.