Filling in marsh areas to create more land is best described as an example of how people
a.
adapt to their resources
b.
modify their resources
c.
adapt to their environment
d.
modify their environment

I would believe that the answer is modify their environment

Since this requires people changing things, they are not adapting (which is to pretty much become used to a change) to anything.

So, that leaves you with B or D. And since the people would not be modifying their resources, they are modifying the environment. That is why I believe the answer is D

Dall of the above all of the choices make sense