Answer:
3. ISDN
Explanation:
ISDN ( Integrated Services Digital Network ) -
It is the used for the simultaneous digital transmission of network service , data , video and voice , instead of the old circuits of the public switched telephone network ( PSTN ) , is referred to as ISDN .
The characteristic feature of ISDN is that it integrates the data and speech on the same lines , which is not possible in PSTN .
Hence, from the given information of the question,
The correct option is ISDN .
