Final answer:
The correct return type of the main function is 'int', which is used to signal the success or failure of a program's execution.
Step-by-step explanation:
The proper return type of the main function is int. In the C and C++ programming languages, by convention, the main function should return an integer value to the operating system upon completion. This return value is often used to indicate the success or failure of the program's execution, with zero generally representing success and non-zero values representing different error states or failure reasons.