The number of cars present at a particular time was measured at 3,999

Where is the question from? Unless the question tells us that no lot had exactly the same number of cars on Tuesday and Wednesday, the correct answer is "cannot be determined".

If we assume that every lot's number of cars changed, we can take advantage of the separation among the answer choices - they're very far apart, so estimation is a great idea, especially with awkward numbers like these. The answer clearly must be less than 1/2 of the total, but not much less, so only C makes sense.

More formally, if a is the number of cars on Tuesday, then 1.15a is the number on Wednesday, and these two numbers add to 3999. So we find 2.15a = 3999, and a is just slightly less than 1/2 of 3999. Since a is what we're looking for, C is the only reasonable answer.

I wouldn't normally do the calculation as above, but the numbers in the question are too horrible to really consider any other approach. If I needed to solve properly, I'd use the fact that if one thing is 15% bigger than another, then the two things are in a 115 to 100 ratio, or a 23 to 20 ratio. If those two things make up a total, then one is 23/43 of the total, and the other 20/43 of the total. So the answer here should be (20/43)(3999). But that's an awful calculation to complete - it's not the kind of thing you ever see on the GMAT. It's worthwhile comparing the numbers in this question to those in OG13-PS Q71, from which this question has essentially been copied. In the OG question, if you solve properly (estimation works well there too), you end up needing to divide 2420 by 11. That's easy enough to do if you notice that 2420 = 121*20 = 11^2 * 20. But here, if you solve properly, you need to divide 3999 by 43. I've never seen a real GMAT question that would require a calculation like that if you solved a question in the standard way.

I think you'll get a much fairer impression of what the real GMAT is like by studying the official (OG) versions of questions like these.