Which one of the following is not a reserved keyword for C?

Answer: Option C

In C programming, reserved keywords are words that have predefined meanings and cannot be used as identifiers (such as variable names, function names, etc.).

Option A: auto - This is a reserved keyword in C, used for automatic variables.

Option B: case - This is a reserved keyword in C, used in switch statements to specify different cases.

Option C: main - This is not a reserved keyword in C. Although main is a commonly used function name for the entry point of a C program, it is not a reserved keyword.

Option D: default - This is a reserved keyword in C, used in switch statements as a default case.

Option E: register - This is a reserved keyword in C, used to define register variables for fast access.

Therefore, the correct answer is Option C: main.