The reason is option D or "music has been proven to calm animals and to make humans smarter." That is because the entire paragraph was about the science to why humans enjoy music so much. Earlier sentences suggested MRI scans, monitoring brain activity, and people being willing to spend more on music based on their brain activity. The final sentence was information about how music calms animals when the entire paragraph was about the science to why humans enjoy music which is why that is your answer.
