Understanding nested yield / return in python

I have a function in python whose output is a generator :

def main_function(x):
    r = get_range()
    for i in range(r):
        yield x+i

I want to refactor the code (I've simplified the use case but actual computation might be complex & longer. Please see EDIT below). Based on my understanding, this is what I should do to keep the functionality unchanged :

(a) Same interface as original code

    def sub_function(x,r):
        for i in range(r):
            yield x+i    

    def main_function(x):
        r = get_range()
        return sub_function(x,r)

As compared to other approaches where :

(b) This would return a generator of a generator (Are there any advantages of this approach ?)

    def sub_function(x,r):
        for i in range(r):
            yield x+i    

    def main_function(x):
        r = get_range()
        yield sub_function(x,r)

(c) This would defeat the purpose of a generator (Is that correct?)

    def sub_function(x,r):
        return [x+i for i in range(r)]

    def main_function(x):
        r = get_range()
        for i in sub_function(x,r):
            yield(i)

EDIT : Comments point out that the right answer is use case dependent. I want to add that my use case is parsing an XML file to extract fields and write them to a database. This part is delegated to sub_function(). I also asked this question for a general understanding of the usage of nested yield for refactoring code.