7 is the answer. hope this helps!
The range of a function is the set of possible values that can be obtained from the dependent variable. The range of the function is: 
Given that:


Let the number of months be t. So, the function C(t) is calculated as follows:



The range is calculated as follows:
The smallest possible value of t is 0 i.e. when no monthly subscription is done.
So, we have:

And the highest is
i.e. for a large value of t
So, we have:

Hence, the range of the function is:

Read more about range of functions at:
brainly.com/question/13824428