if create a object which has instance variable, inside a method then it will exist in heap memory or stack?
You need to understand the difference between an object and a reference to an object. An object is always on the heap (there is an exception, but let's not get into that ...
). A reference, can be on the heap, if it is part of the object, such as an instance variable. Or it can be on the stack, when it is declared locally, such as a local variable or a parameter.
Now as for your question...
There is no such thing as an instance variable inside a method. Variables declared inside a method are local variables.
Henry