Documentation: Something that provides information on something
Conclusion: A computer is specific so it won't understand just from the documentation like we humans.
Variable declaration: The process of declaring a variable
Conclusion: What? How is this in the options?
Compilation: The action or process of producing something.
Conclusion: The answer is already given so don't even look.
Execution: The initiate a defined plan ("To follow orders for example.")
Conclusion: This and compilation can be confusing but programs execute themselves only after the operating system has finished compiling the source code.
So? Compiling is all about translating the code into machine language for the computer to understand. A compiler is what does compiling. Not all programming language work the same way so different kinds of compilers are needed for different languages.