How will the right answer fit in terms of support and conclusion?

Only the right answer will add support for the conclusion.

Highlight the main conclusion in the passage, if there is one:

the mountain snowpack will probably melt more rapidly and earlier in the season, leading to greater spring flooding and less storable water

Climatologist: [BACKGROUND]. [SUPPORT]. Therefore, [CONCLUSION].

That’s one wordy conclusion, but the cause-and-effect thing the author is claiming should be fairly clear. They’re predicting what this whole chain kicked off by “global warming” will lead to. To “strengthen” that, the right answer will definitely map to that same chain of cause-and-effect.

Map the wording of the answers to the wording of the passage:

(A) …increase in the average amount of annual precipitation…

This doesn’t say anything about the snowpack melting OR the greater spring flooding, so it really doesn’t map to the cause-and-effect we want to support. Also, lawyers will note that the passage talked about the “proportion” of precipitation that’s rain vs. snow, but the “average amount” of precipitation never comes up. Those are not the same.

(B) …after relatively mild winters, the melting of snowpacks has led to greater spring flooding…

I’ve had many people tell me they eliminated this one because they thought “other mountainous regions” was irrelevant. But what does the passage say kickstarted the cause-and-effect chain? “global warming”. They expect you to know that means the same effect would be happening in other regions.

Otherwise, this wording maps just beautifully to the background about warmer winter temps, and to both parts of the conclusion. If this is true it definitely supports the cause-and-effect the author claimed in the conclusion. Perfection.

(C) On average…there is less storable water…

Again, “on average” is a different kind of data from what’s mentioned in the passage. More critically, this answer jumps to the “less storable water” bit but doesn’t say it’s because of the snowpack melting, so it doesn’t really align to the cause-and-effect in the conclusion. And if that didn’t bother you, then the comparison between different areas of the Rocky Mountains should have, since that doesn’t map to any comparison made in the passage.

(D) On average, in the regions of the world with the mildest…

“On average” again I don’t like. But also be careful with what this says: “mildest winters” is changing the subject. The passage only says the temps are “likely to increase”, but doesn’t say anything like mild-est. Answers that use superlatives and extremes like that will only map to passages that use the same extreme wording.

(E) The larger a mountain snowpack is…

Please stop reading at “The larger…” There is no mention of the size of the snowpack, let alone whether it’s getting bigger.

(B) is the correct answer.