1.If You Forget to use '&' operator in scanf, then you will face this error.
2.While using 2D array ,like your declaration if you also put '**' in Scanf and Printf braces , then you will have this error.
2nd--Implicit Declaration
Reason:
* If you have used a different keyword in ur program which ur current header file doesn't support or defines it ,then you need to include your keyword by its appropriate header file. Otherwise you have error of Implicit Declaration.
Example: Suppose you are using the keyword exit like below 'exit(EXIT_SUCCESS)' Then if you haven't include it's header file #include 'stdlib.h'