bipolarbear, I know you don't want to hear "it is not idiomatic", and I know my scores are not higher as well, but here is my attempt.
A It is missing the pronoun. We don't know what will be kept.
B CORRECT Adds the pronoun.
C My attempt is that you keep what it is said in the Kaplan 800
book. - I don't remember the right words, but says something like this: "Idiomatic expressions don't follow rules, they are what they are. They exist and you need to get used to them and memorize those that you don't know...." So sorry, the explanation is "it is an incorrect idiom".
D in order... unnecessary
E in order... unnecessary
That is why there are lists and lists of correct idioms to memorize for GMAT.
In this thread there are some of these lists. new-to-the-verbal-forum-please-read-this-first-77546.html
If you liked, consider a kudo