Remember that we don't need to disprove the conclusion; we just need to cast doubt. We don't know what importance, if any, the retention of star faculty has in maintaining the school's ranking. D tells us that something else is the MOST important. And, importantly, one part of that factor is economic--the school will benefit from attracting a wide range of students from different socioeconomic backgrounds. So not only might they benefit from spending their money on something else (scholarships, programs to increase diversity), but they might actually lose lower-income students if they raise tuition by 10%. This certainly casts doubt on the effectiveness of the plan. Could it still be a great idea? Sure, given more info, it might turn out to be the absolute best thing to do for rankings. But with what we know now, it doesn't look promising.
Pranavsawant
I am a bit iffy on this. "The single most important factor" does not mean the only factor as most people are saying in the comments. Which is why I was very concerned when picking D. The second most important factor could be quality of faculty and that could very well protect/diminish the uni's ranking. It could be the case that the uni has a very high diversity already and instead needs to work on their faculty strength in order to protect their ranking. Too many holes option D has imo