The median is the number in the middle when they are arranged in order. If the median is 4.5, there will be an even amount of numbers and the middle will be in between 4 and 5. This means that one of the numbers 6-9 needs to be taken out. The only answer choice that meets this is D. 6
If you rewrite the numbers without the 6 you'll get 3, 3, 4, 4, 5, 7, 8, 9 and the median is 4.5.