Final answer:
The primary function in a C program that is referred to when executing the program is the 'main' function. This function serves as the entry point and must be present in every C program.
Step-by-step explanation:
When a user requests to execute a "C" program file, the primary function referred to is the main function. The correct answer is a) Main. In C programming, execution of a program starts with the main function, which is the entry point of any C program. It is where the execution of the program begins, and it must be present in every C program. This function can call other functions as needed, but it is always the starting point for execution.