In order to find the correct answer in this one, you have to group all of the like terms. In this case, each term is unique with the exception of the x^2y^3 terms. Therefore, we need to check those first.
4x^2y^3 - (-5x^2y^3)
4x^2y^3 + 5x^2y^3
9x^2y^3
Since this term is included, we look for answers that involve this one. A is the only one that does. Plus, it includes each other term unchanged (which is good since there are no other like terms). Therefore, A is your answer.
