One way that humans could have a positive impact on local environments is to

Let's first think what could be a "positive" impact - this would mean making the environment more clean and more friendly towards other species. 

So they could clean up the environment (from the mess that was likely introduced by a human activity) and support other organisms in it - for example by removing the physical barriers for migration of other species (bridges over the streets for example) 

Icould be wrong but i would say b.