Fire ants from Brazil now infest the southern United States. Unlike queen fire ants in Brazil, two queens in the United States share a nest. Ants from these nests are more aggressive than those from single-queen nests. By destroying virtually all insects in the nest area, these aggressive ants gain sole access to food sources, and the ant population skyrockets. Since certain predator insects in Brazil limit the fire-ant population there, importing such predator insects into the United States would be of overall benefit to the environment by stopping the increase of the fire-ant population in the United States.
Each of the following is an assumption made in the argument EXCEPT:
(A) The imported insects would not prove more damaging to the environment in the United States than are the fire ants themselves.
(B) The predator insects from Brazil could survive in the ecological environment found in the United States.
(C) The especially aggressive fire ants from the two-queen nests would not be able to destroy the Brazilian predator insects.
(D) The predator insects would stop the increase of the ant population before the ants spread to states that are farther north.
(E) The rate of increase of the fire-ant population would not exceed the rate at which the predator insects could kill the ants.

i will go for D as well

The main conclusion is : Since certain predator insects in Brazil limit the fire-ant population there, importing such predator insects into the United States would be of overall benefit to the environment by stopping the increase of the fire-ant population in the United States.

So lets work backwards and see what would happen if we take away the assumption :
A The imported insects would not prove more damaging to the environment in the United States than are the fire ants themselves.
Pretty obvious that this is a required assumption

B The predator insects from Brazil could survive in the ecological environment found in the United States.
If they didn't they wouldn't be able to limit fire ant population - so back to square one

C The especially aggressive fire ants from the two-queen nests would not be able to destroy the Brazilian predator insects.
If this were the case then predators would fail and again back to square one

D The predator insects would stop the increase of the ant population before the ants spread to states that are farther north.
So what ?? That doesn't mean that predators wont stop the increase of the ants eventually

E The rate of increase of the fire-ant population would not exceed the rate at which the predator insects could kill the ants.
In this case fire ants predators would still be ineffective

thanks +1 bsd