My answer is
(D). It took me 01:37
(A) The meaning is rather vague. Reading from left to right (as almost every reader does), we get the feeling that "additional volatility in prices" is due to the fact that "it takes so long to grow crops". However, it is more logical to believe that farmers must fix next year's output based on current prices because it takes so long to grow crops. And farmers' action cause additional price volatility. This sentence fails to convey the meaningful relationship between three facts.
Also, passive voice “additional volatility in prices is caused” also counts against (A).
(B) Like (A), this version is also rather unclear in its meaning. The use of passive voice (including "crops are grown for so long") won't help.
(C) While the meaning is much clearer than (AB), (C) suffers from three issues:
(1) "it takes so long to grow crops" starts the chain of events. Ideally, it should be described at the very beginning of the sentence.
(2) Who is "fixing next year's output?".
(3)Passive voice.
(D) A pleasure to read this version. Meaning is clear and logical. Almost certain this is the correct answer.
(E) This version is pretty decent. Even though it does not specify who is fixing next year's output, such omission is not fatal. Not placing because at the beginning of the sentence is also well within the control of its author. So, what's really wrong with (E)?
I would say that there is some ambiguity in fully comprehending the present participle phrase after the comma ("causing additional volatility of prices"). It can either show the effect of "Fixing next year's output on current prices is necessary" or the impact of "it takes so long to grow crops". Either is plausible to a casual reader, and that is the problem.