All of the following types of admission policies have a deadline except a) rolling admission b) late admission c) regular admission d) open admission

The best answer here is rolling admission. Rolling admission means that you can apply whenever you'd like and you'll find out if you are admitted once your application is received and reviewed.

For late admission and regular admission, colleges will require a due date to ensure they are able to sort through all of the applications they receive before the deadline. Open admission deals with the requirements that a college requires for a student to attend.

The meaning of the word in this sentence is unneeded