Which C++ header files are essentially required to be included to run/execute the C++ source code (Note. Do not include any header file, which is/are not required)

Answer 1

iostream is pretty much the only mandatory one.

Answer 2

I am guessing you are refering to libraries, an there is no such thing as a required library.

You include headers based on the functionality you want to implement.
For example, if you want console output, you should include iostream, but if you don't need to interact with the console, there is no need to include it.

Answer 3

If you are writing a GUI appplication or an application which reports status via TCP, iostream is useless since you don't use the console.