"What line of code can be used to provide a convenient way to merge the source code from one file with the source code in another file, without having to retype the code?"

The #include directive causes the source code of a second file to be inserted into the original file without having to retype the code.

Explanation:

Include is a directive that many programming languages and computer files use in order to merge the source code of one file with that of the another file without retyping the code

# include is used in programming languages like c,c++ because it includes the library functions which makes coding simpler .

For example

#include<Math.h> - is responsible for the mathematical calculations if we don,t  use math.h then we will face a error in the code