Because this is an assumption question, we can eliminate any answer that doesn't HAVE to be true for the conclusion to be valid. An assumption is a pillar without which the argument would fall apart.

The premise claims that the established companies won't be adding as many jobs. The argument is that the only way to get a record # of new job iss if there are a record number of new companies. The logic gap is that there is the assumption that it would be impossible for only a few companies to each create so many jobs that we will still have a record # of new job even though we don't have a record # of companies.

C identifies the assumption and is therefore correct.

To look at a detailed solution, please review GMATPrep question 2027. You can find similar questions by searching for topic='Assumption' and difficulty='600-700' in the Solutions Engine.

Good luck,
-Patrick