this one is irrelevant to the stimulus: Ensuring that

Looking at the extent to which the Professors/group of professors can make a difference to the students, we can eliminate the choices.
The argument mentions that the professors can edit and add material to the textbook according to their own liking. Since changes made by these professors will benefit only the students they teach, this kind of becomes a localized phenomenon. A professor can add or edit material according to the needs of the students he teaches, or a group of professors can customize the textbook according to the needs of the student body/elective course or university students that they teach.

Students nationwide are not likely to benefit from this phenomenon since various professors nationwide will bring out various versions of the textbook, and it will be left to the students to choose among the thousands of versions available.

So C seems to be the most apt choice