Thanks for the question! Let’s take a look at the stimulus. Administrators of educational institutions are excited about using computers, whereas teachers fear computers because they might lose their jobs to the computers. The author says that those teachers are mistaken, since computerized instruction requires more instructor time.
The question asks about how the premise about computers letting schools teach more classes with fewer teachers figures in the argument. Well, we know that this is the idea the author opposes; the author thinks that more computers will actually require more teacher time, as the author states in the last sentence. Thus, (D) cannot be right, as the author is opposed to (D), not using (D) to support her argument.
Hope this helps! Feel free to ask any further questions that you might have.