What should be the correct answer of hello?

Answer 1

either hello, good day, hi, howdy, g'day mate, or any other form of saying hello.

If you want to be rude, then say goodbye, go away, or any other form of telling someone to leave you alone.

Answer 2

Assuming the 'hello' is not being used sarcastically, like 'hello?' so it is a question rather than a greeting, then there is no right or wrong answer. 'Hello' is fine as is 'Hi' or 'How are you?' - just be friendly back...