Final answer:
ISDN is considered to be a circuit-switched connection.
Step-by-step explanation:
The correct answer is b. Circuit-switched connection.
ISDN stands for Integrated Services Digital Network. It is a set of communication protocols used for transmitting voice and data over digital telephone lines or other communications networks.
ISDN uses circuit-switched connections, which means that a dedicated physical connection is established between the sender and the receiver for the duration of the communication. This is different from packet-switched connections, where data is divided into small packets and sent separately over the network.