This is a hard one and I can see why you decided to post it here. B is most definitely wrong but the other three are all neck and neck. There are both even pros and cons of A and C but I think it would be more reasonable to adopt a general routine. So I think the answer would be C.
They're. They are driving to Yellowstone National Park tomorrow. Theirs doesn't sound right in the sentence. Their means belonging to. So they're fits in the sentence the best.
This mostly depends from country to country, but most commonly people such as teachers, accountants, often doctors, builders and civil engineers, basic lawyers are often found as parts of a saturated job market.
