primary like you said, champion, chief, consummate, expert, finished, first, foremost, greatest, head, highest, leading, main, major, master, preeminent, premier, primary, superior, top, main, basal, basic, beginning, bottom, central, elemental, elementary, essential, first, foundational, introductory, meat-and-potatoes, original, prime, primitive, principal, radical, rudimentary, simple, three R's, ultimate, underivative, underived, underlying, capital, cardinal, chief, crackerjack, dominant, excellent, fab, first, first class, greatest, heavy, highest, hot, leading, main, number one, numero uno, paramount, prime, primo, principal, stellar, ten, top, tough, world class.

Sorry, I may have gone overboard, copied it from the thesaurus.  I would agree that it is the primary source of lift, as most general aviation, traing aircraft don't implement lifting bodies.

Good Luck!

Mobius

I think the point of this question is to get you to not get sidetracked pondering Bernoulli's Theorem or any other such subtleties of why that differential is created; just to keep you focused on what the wing is doing. Personally, I believe that the air being forced down and back (as described by Langeweische) has just as much to do with it (if not more), but the FAA is really keen on the pressure differential and holds Bernoulli's idea as gospel, so in this case "primary" is the "right" answer.
And while it's important to do well in ground school, remember that none of that does you much good in flight. There are no pressure gauges above or below the wings, unless you count the stall warning horn- but a competent pilot knows a stall is imminent before the horn is heard.

Yep. I recommend you do what i did: learn the answers "they" want to hear, but secretly study Langeweische, who taught me more about flying than my ground instructors (who were good flight instructors and simply trying their best, to give credit where due). When I started flying on my own, his words always came to mind, whereas the official doctrine didn't seem to fit when it came to what was holding the plane up, and why it did what it did.

A quote from Stick and Rudder, which is a must for any aviator's library:

"What is wrong with 'Theory of Flight', from the pilot's point of view, is not that it is theory.... it is the theory of the wrong thing... building the airplane rather than flying it. It goes deeply-much too deeply for a pilot's needs- into problems of aerodynamics... but it neglects those phases of flight that interest the pilot most....
This whole book... is an attempt to refocus the 'Theory of Flight', away from things that the pilot does not need to know about, and upon the things that actually puzzle him while he flies."
  Learn your textbooks backwards and forwards, but check out Stick and Rudder, too.