There are a few ways to solve this problem. You can use algebra, but you will quickly find that it degenerates into a fairly nasty quadratic equation.
Easier than that is to just try the answer choices and see what works. Since answer choices are almost always in either descending or ascending order, it's often a good strategy to start with C, since if C is too small, we can pick a larger value, and if it's too large, we can pick a smaller. However, picking B or D is actually a little better, provided the numbers aren't harder than C.

Let's try B first. If every cookie were 3 ounces originally, then there must've been 600/3 = 200 cookies originally. If we decrease the weight by 1 ounce to 2 ounces, now there must be 300 cookies. This is an increase of 100, so B is wrong. But is it too big or too small? Well, since we started with a small sized cookie, a change of 1 ounce made a pretty big difference, so picking a larger size should make a smaller difference.

Let's try D. If the original weight were 5, then we started with 120 cookies. Decreasing the weight to 4 gives us 150 cookies. Since this is an increase of 30 cookies, this must be the right answer. D it is.