Pls note the question first: "This argument is valid only if....."
This means, the above argument is valid only if the assumption is true.
And the funda here is 'Can we irrigate the corn fields now, to avert inflation in the future?'
Now lets tackle the options,
(A) Firstly there is drought and because of that there is a 33% decrease in the year's corn yield. This option leads to further decrease in the corn yield saying the other American agricultural areas are also not expected to meet their anticipated corn yields. Thus, cannot be the answer.
(B) This option sounds right the fiirst time but is not fully true. The word 'only' infact tends to lead this answer being incorrect.
(C) PR style: This would have been the Joe Bloggs answer. The heat wave doesnt have to persist for two years. The inflation of 17% happens in the first year itself.
(D) This option is very similar to Option (B) but without the word 'only'. This option is true, but I would rate it the second best option in the list.
(E) The only option left out. Now, try looking at the argument from the author's point of view - the reason for him to write this passage.
This option supports the conclusion, and only if this is true, can the rest of the story progress.