The correct answers are:
15. carbon dioxide;
It is thew gas that is the most responsible for the Greenhouse effect by managing to trap the heat in the atmosphere.
16. introducing exotic species into new environments;
The introduction of invasive species is damaging to the biodiversity as those species are often out-competing the native species and systematically destroy them.
17. hunting a keystone species to extinction;
Hunting a species to extinction is directly contributing to the destruction of the biodiversity, not in preserving it.