[quote="Bunuel"]Since stock exchange indexes fluctuate unexpectedly, every buyer should be aware of the dangers of disappointing investments and its consequences.
A. fluctuate unexpectedly, every buyer should be aware of the dangers of disappointing investments : First half of the underlined portion looks good. It is stated in Simple Tense, which denotes a fact/habit of stock exchanges, so the verb usage is correct here. But in the end, in the non-underlined portion, one should pay close attention to the usage of "its", this implies the noun that "its" is referring to must be "Singular", so "Disappointing Investment" is the correct usage here. This way you can simply eliminate options A, B, and C.
B. unexpectedly fluctuate, buyers should be aware of the dangers of disappointing investments : Fluctuate unexpectedly is better than unexpectedly fluctuate. Also, it has the pronoun error as discussed in Option A above.
C. can fluctuate unexpectedly, every buyer should be aware of the dangers of disappointing investments : Can fluctuate distorts the meaning of the original sentence. The author wants to present a general behavior or factual nature of stock exchanges. He/She does not want to convey that stock exchanges have the "ability" to fluctuate, which is being implied by the usage of "can" here. So, this usage is incorrect. It also has the pronoun error as discussed above in Option A.
D. fluctuate unexpectedly, buyers should be aware of the dangers of a disappointing investment : This option uses correct verb tense and conveys the logical meaning implied by the author. Also, the singular "disappointing investment" is the correct antecedent to "its" in the non-underlined portion.
E. may experience unexpected fluctuations, every buyer should be aware of the dangers of a disappointing investment :The usage of "may" again distorts the meaning. The author doesn't want to convey that the stock exchanges may or may not fluctuate, instead, the author wants to speak about their general nature. See, this option has correctly rectified the pronoun error. But the usage of "may" distorts the meaning. Hence, this answer choice is incorrect.
Option D, thus stands out as the correct choice.
Take away from this question would be: Take help from the non-underlined part of the question to eliminate wrong answer choices.