1 – Wrong
We do not know for sure whether the less developed countries have set targets or not, so we cannot consider this statement as an assumption.
2 – Wrong
The text does not mention whether developed countries are prioritising alleviating poverty or reducing global warming.
3 – Wrong
We do not know whether levels of prosperity in developing countries are improving or not.
4 – Correct
The text gives two options: either reduce global warming or alleviate levels of poverty in developing countries. So it must be assuming that we cannot alleviate poverty without contributing to global warming. So the assumption is that developing countries can only improve their economies by activities which generate greenhouse gases.
Since only statement 4 is correct, B is the answer.