When the manufacturer of Voltage, a major soft drink, changed its secr

Conclusion: Vanilla from beans was one of the ingredients in the original formula but not in the new formula.
We need to focus on the conclusion as that needs to be strengthened.

(A) The vanilla-bean plantings of a nearby island were beginning to produce crops.
Not relevant. We don't need to focus on nearby islands.

(B) A new process for synthesizing vanilla was under development in a laboratory in the United States.
Not relevant. As per the conclusion, vanilla is not being used hence it does not matter how it is being developed. Another indicator is the mention of the USA whereas no such mention is present within the passage.

(C) The island's trade agreement, under which the vanilla beans were exported to the country that manufactures Voltage, had lapsed.
Since the formula has been changed manufacturers don't need vanilla from beans anymore. Additionally, if the trade agreement would have expired it would have meant that the manufacturers were forced to change the formula but from the passage, it is evident that they changed the formula and after that, the exports were impacted and not the other way around. Hence incorrect.

(D) Imports of vanilla beans dropped in countries where Voltage is made.
This complete the logic within the passage. If the export of beans from the island has reduced it logically follow that wherever these beans were being exported, import of those countries should reduce and hence this option is correct.

(E) There were decreases in sales of several widely sold products that were known to contain vanilla.
This takes the whole argument in a different tangent altogether. Not relevant and broadens the scope.

Hence D is the correct answer.

vmkumar, hope this helps with your doubt.