What is ISD in telephonic communication?
A. Inter Subscriber Dialing
B. Inter Secure Dialing
C. International Subscriber Dialing
D. Intercom Simple Dialing

Hint:
The above problem is resolved using the concepts and the fundamentals of the STD and ISD calls. The ISD calls are meant for placing the international calls or the calls that are prominent to the outside countries. The ISD calls comprise of ISD prefix, ISD code and the Area code or the phone number. These things must be considered while placing international contact.

Complete step by step solution
We usually come across the telephonic terms like ISD and STD. The full form of STD is Subscriber Trunk Dialing. Similarly, the ISD has its full form as International subscriber Dialing. The ISD is one of the crucial communication modes in which the call is directly placed to the foreign or the international countries through the telephone subscriber. In the process of making an International call, the initialisation of the calls starts with dialling the international prefix call for the residing country, and this is followed by the country calling code for the designation country. And at last, the national telephone number of the designation is dialled, to make a successful contact in international countries.
Therefore, the ISD means International Subscriber Dialing and option (C) is the correct option

Note:
To resolve the given problem, one must understand the meaning and major differences between the STD calls and ISD calls. STD calls are useful to contact in local residence or to place any call within the country. While on the other hand, ISD meant for placing any call outside the nation.