The best answer is B) Native plant populations will decrease as they are choked out by the purple loosestrife.
Two results are likely when an invasive species, plant or animal, is introduced to a new population: that species will compete very well in the new environment or will not be successful.
One possible result of the introduction of the purple looseleaf is that the species will do very will in its new environment, out-competing native plant species and thus choking them out of their natural environment.