When a certain tree was first planted, it was 4 feet tall

[zeshan.dhanani]

You could also try plugging in answer choices, and it is fairly quick. Usually, when plugging in, start with C, b/c many times you are able to determine whether it is too big or too small and consequently knock-out two other choices as well.
So start with C) 1/2 feet. The heights are then 4, 4.5, 5. 5.5, 6, 6.5, and 7. At the end of year 6, the tree was 7 feet, or 1/6th taller than at the end of year four. Since it is only 1/6 taller, this growth was not large enough. So cross off A, B, and C.
Test 2/3. You get 4, 4 and 2/3, 5 and 1/3, 6, 6 and 2/3, 7 and 1/3, and 8. 8 is 1/5 taller than 6 and 2/3, so you have the correct answer, D.

[jlucero]

I like it. Especially when the computation is straightforward, adding and multiplying simple numbers can be a lot easier/faster than coming up with algebraic equations.