Question: What does this part of a main.cpp file do?

1. It causes the toolchain to compile all the contents of library.h so that its executable code is available when needed by the final application.
2. It cherry picks library.h for the declarations and definitions of all data and functions used in the remainder of the source file main.cpp, finally replacing the `#include` directive by those declarations and definitions.
3. It informs the linker that some functions or data used in the source file main.cpp are contained in library.h, so that they can be called in run time. This is also known as dynamic linking.
4. It causes the replacement of the `#include` directive by the entire contents of the source file library.h. This is similar to a Copy-Paste operation of library.h into main.cpp.

Answer: The correct answer of the above question is Option D:It causes the replacement of the `#include` directive by the entire contents of the source file library.h. This is similar to a Copy-Paste operation of library.h into main.cpp.