In Ponoma College, a rule has been passed that permits students to coo

I'll go back to my old mantra here and say that you want to strive to identify the type of problem (based on the signal words in the passage) and then use what you know about that type of problem to make your decisions. In this problem, we have a list and the word "and" so we're clued in to this being a parallelism problem. So let's look for parallelism problems:

In Ponoma College, a rule has been passed that permits students to cook and serve their food, as well as to buy it.
A. permits students to cook and serve their food, as well as to buy it. - no parallelism problems here
B. permits students to cook, serve and to buy their food. - parallelism problem. You can't have the second "to" there since you don't have "to" before "serve."
C. permits students to cook, to serve and buy food. - parallelism problem. You can't have two items in the list with "to" and the third one without.
D. will permit the student to cook, serve as well as to buy food. - parallelism problem. If you want all three items to be parallel (which you do), you can't have the "as well as" there.
E. will permit food to be cooked,served as well as bought by students - parallelism problem. Same reasoning as above. The passive voice thing isn't technically a problem. It doesn't "sound right," but grammatically the passive voice isn't necessarily wrong.

When you do all problems, you should look for as many errors as you can so you learn as much as possible. However, when it comes right down to it, realize that the GMAT (and most if not all tests) write questions to test a certain principle or set of principles. If you can identify what they're trying to test, you have a leg up on your competition.

Brett