Final answer:
ISDN (Integrated Services Digital Network) is the set of communication standards for digital telephone connections, allowing for the simultaneous digital transmission of various services over telephone networks.
Step-by-step explanation:
The type of Internet connection that is a set of communication standards for digital telephone connection and the transmission of voice and data over a digital line is c. ISDN (Integrated Services Digital Network). ISDN allows for the simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. It was once a common method of Internet access before being widely replaced by technologies such as DSL (Digital Subscriber Line) and fiber optic connections that provide higher speeds and better quality data transmission. However, ISDN still plays a role in some business and telecommunication applications where dedicated lines are important.