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

Only the right answer will accurately describe the support and/or conclusion in the passage. It’s more precise to say only the right answer will call out why this support doesn’t establish this conclusion, but there’s no need to be that specific.

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

this year in Browning more rain will probably fall in September than in July.

[SUPPORT]. Therefore, [CONCLUSION].

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

(A) contains a premise that presupposed the truth of the conclusion

This answer choice shows up pretty regularly, and it’s pretty much always wrong. If an argument “presupposed” its conclusion, it would sound like ‘it will rain more in September because there will be more rain in September’. If the support and conclusion aren’t the same, then this isn’t the right answer.

(B) draws an inference about a future event on the basis of a very limited number of instances…

Is the conclusion about a “future event”? Yes it is. Is it based on “a very limited number of instances”? Yup, just the last two years. This all maps, so you’re keeping this one in the running for sure.

(C) …average rainfall statistics could be skewed…

There’s nothing about “average rainfall”, so no way the author “overemphasizes” anything about that.

(D) …there are many instances in which both phenomena are present

It would be a stretch to make the first part of this work, but the passage definitely does NOT make this claim. Months of the year are not “phenomena”.

(E) …drawn from a source whose reliability cannot readily be verified

An answer doubting the source of info only works when the source is actually mentioned in the passage. You don’t know if the “reliability” of the author’s source on rainfall data can be verified or not, and the LSAT says you’re not supposed to add in your own assumptions.

(B) is the correct answer.