Assess the effect of the introduction of a nonnative species of tree on the biodiversity of a forest.

The answer is the option c. fact. Indeed facts are the events such as they occur, without interpretations or opinions. Facts happens and they can be observed by different means: you can get them through sense organs (eye, ear, mouth, skin, nose) or with instruments. When you report facts you must clearly state them appart of your interpretation or opinion.