https://www.gregmat.com/quizzes/quiz/math-fundamentalsbasics-quiz-2-level-1 (Question 6)
The correct answer should be A (as the solution suggests and according to my calculations) but the test has marked D as a correct answer if I am not mistaken
Not quite.
That is the correct answer. There are notes on the video to this effect.
To understand why D, try n=0
thank you, understood