I am going to buy the book even though I changed my mind about buying the book?
"am going to buy" is answer D. I suggested answer C ("was buying").I am going to buy the book even though I changed my mind about buying the book?
I think that only works all in present tense as if telling a story.
I am going to buy the book when I hear what the critics say about it and I change my mind.
I handed three books to the clerk with a hearty "Hello, Geoffrey! I think three is my limit today. How are the children?"I was buying the book but when I heard what the critics said I changed my mind.