This question appeared in some Mock GMAT

The time it takes for a star to change its brightness is directly related to the luminosity of it.
a. the luminosity of it
b. the luminosity of its brightness
c. the luminosity of a star
d. luminosity of it
e. its luminosity

The answer is E. Here is the explanation
The original version is grammatically correct, but the pronoun reference is vague. (To what does it refer?) (E) clarifies the pronoun reference by using the possessive its luminosity.

Isn't 'its' in option E also vague as it can refer to both "star" and "brightness"?