Since all the other questions that you asked today were about Collapse, I assume that this question is about that book as well. I will answer using quotes from the book, but my answer will be the same as the other answer. That is, Option C is the option that is not a cause of water scarcity in the Bitterroot Valley.
In Chapter 1 of Collapse, Diamond goes through a list of factors that cause water scarcity in the Bitterroot Valley. I only have this book on Kindle and therefore cannot give you page numbers where these factors are found. The first factor that Diamond mentions is global warming. He says
The ultimate reason for decreasing amounts of water is climate change: Montana is becoming warmer and drier.
Diamond goes on to talk about how this change has been very obvious in Glacier National Park, where glaciers have been shrinking and even disappearing altogether. This means that Option D is an actual problem and is therefore not the right answer.
After discussing global warming, Diamond turns to the next factor. Here, he says
…the water is “overallocated.” That is… the sum of the water rights allocated to all landowners exceeds the flow of water available…
Diamond goes on to say that this overallocation is due in part to the rise of new residential subdivisions to go along with all of the farmers who need to irrigate. This means that Option B is not the right answer.
Finally, Diamond talks about the number of wells being drilled. He says that a problem is the problem of
…more wells drilled into the same aquifer and lowering its level.
Diamond says that this is exacerbated by the laws in Montana which do not regulate the impact of wells drilled by one person on existing aquifers. This means that Option A is not the correct option.
Thus, we can see that Diamond cites three of these four options as factors that cause water scarcity in the Bitterroot Valley. That means that the remaining option, Option C, is the correct answer.