The correct option is B devastating
Option A: 'Decreasing' means 'to make or become smaller in size or amount'. For example, the 'decreasing population'.
'Decrease' is generally used for a quantity or an uncountable noun, 'population' is a mass noun, for example, and hence cannot be counted. In contrast, the given sentence contains a number for the kangaroos, 'more than 1500'. Hence option A is incorrect.
Option C: 'Deriding' means 'to express contempt for, to ridicule'.
Option D: 'Defying' means 'going against, openly refusing'.
The words 'deriding' and 'defying' are irrelevant to the given statement. Consider: kangaroos ridicule the environment, or kangaroos refuse to the environment. Hence options C and D are incorrect
Option B: The word 'devastating' means 'highly destructive or damaging'. It's the most suitable option, as it completes the given sentence in meaning and logic. The sentence states the reason behind the government's plan, "as they have a devastating effect on the environment'. Hence B is the correct option.