The C #if, #elif, and #else directives are preprocessor directives in the C programming language. They are used to conditionally compile sections of code, depending on the evaluation of certain expressions. Here is an explanation of each step:

1. if Directive:

   The #if directive allows you to conditionally compile a section of code based on the evaluation of a constant expression. If the expression evaluates to true (non-zero), the code within the #if block is compiled. Otherwise, it is skipped.

2. elif Directive:

   The #elif directive is used in conjunction with the #if directive to specify an alternative condition if the previous #if condition is false. It allows you to provide multiple conditions to choose from. If the expression following #elif evaluates to true, the code within the #elif block is compiled. Otherwise, it is skipped.

3. else Directive:

   The #else directive is used to specify an alternative code block to be compiled if all the previous #if and #elif conditions are false. If none of the conditions in the preceding #if and #elif directives are true, the code within the #else block is compiled.

It's important to note that these directives are evaluated at compile-time and not at runtime. They are mainly used for conditional compilation, allowing different parts of the code to be included or excluded based on specific conditions. This can be useful, for example, when dealing with platform-specific code or when enabling or disabling certain features based on configuration options.