What are the instructions that control the changes introduced by the preprocessor called?

The correct choice is 'Directives.' In C programming, preprocessor directives are instructions that are processed by the preprocessor before the actual compilation of the code begins. These directives typically start with a '#' symbol and are used for a variety of purposes such as including files, defining macros, and conditional compilation.

For example, the #include directive tells the preprocessor to include the contents of a specified file into the program. Similarly, #define creates a macro that can represent a symbol or a constant, while conditional directives like #ifdef and #ifndef control the inclusion of code segments based on certain conditions.

The term "commands," while it might seem fitting, doesn't accurately describe how these directives function in the context of the preprocessor. Likewise, "parameters" generally refer to values that are passed to functions. "Modifiers" typically relate to keywords that change the properties of variables, functions, or classes. Understanding preprocessor directives is crucial as they influence how the source code is modified before the compilation stage, impacting the final output of the program.