is the main() in c is user defined or predefined function?

By the language point of view, it is the function that the program start-up code calls after global initialization finished, so it is "user defined".

Whether it is provided by the user itself, into its own source file, or linked from some other library (like MFC, for example) depends on what environment the user is working with.

The question seems formulated as a (sort of) trap, since it does not define what "user" means.