The first thing that should be considered is alphabetical precedence.
If a term has x in it, it should be considered before y.
Then sort by powers. For example, x^3 should go before x^2.
Finally sort by coefficient if the variables and their powers are the same.
A is incorrect because 8x^7 should go before all else.
B is incorrect because 4x^4 should go before -3x^4.
C is incorrect because the order should be 4x^3, then 9x^2, then 6x
D is correct.