What will happen if the C program is executed without including the proper header file?

When a C program is executed without including the proper header file, the most likely outcome is a compile error. Header files provide the necessary declarations for functions, macros, constants, and other elements that the code depends on. For example, if a program tries to use functions from the standard library, such as printf or scanf, but does not include the relevant header file, the compiler will not recognize these functions. This results in errors during the compilation process because the compiler does not have enough information to understand the functions being called.

In this context, compile errors indicate that there are references in the code that cannot be resolved, leading the compiler to provide error messages indicating the specific areas where it lacks proper declarations. Therefore, ensuring that all necessary header files are included is crucial for successful code compilation and execution.