The best part of C is that the answer is conspicuous but you have to think through them all without any bias.
(A) The vanilla-bean plantings of a nearby island were beginning to produce crops. It could be possible that the nearby island also supplies vaniall beans but that doesn't mean Voltage contains Vanilla beans.
(B) A new process for synthesizing vanilla was under development in a laboratory in the United States. Underdevelopment , an underdevelopment thing is in a nascent stage and cannot change an export of a commodity.
(C) The island's trade agreement, under which the vanilla beans were exported to the country that manufactures Voltage, had lapsed. The agreement being lapsed will lead to fall in the exports but that doesn't mean Voltage has Vanilla
(D) Imports of vanilla beans dropped in countries where Voltage is made. There we go , if imports are dropped where voltage is made clearly shows that something is wrong because it drooped for all the countries . So Vanilla must be there in Voltage
(E) There were decreases in sales of several widely sold products that were known to contain vanilla. Opposite of what we want