Which activity would reduce the biodiversity in a forest ecosystem

Deforestation might harm the habitat of the organisms who live there, causing these organisms to die or be forced to relocate.

While you didn't really provide a list of possible answers, the things that would reduce biodiversity in a certain forest ecosystem would be for example deforestation, intentional killing of species through poaching or selective hunting, exploitation of resources which are also used by other animals from an ecosystem, etc. All of these activities have important roles in reducing biodiversity in forest ecosystems and are generally considered to be bad.