This is an explanation question. The apparent paradox is that, while the price of rice effectively declined, rice consumption also declined. Consider each choice in turn, looking for a reason why this might occur:

(A) This doesn't tell us anything about the effect of the coupons.
(B) The amount of fluctuation is outside the scope; we're concerned with the effect of lowering the price of rice.
(C) That the average consumption of rice declined is not important; the study described in the passage included a control group, to which the households given coupons were compared.
(D) This choice suggests that some families did not purchase rice with their coupons; however, this doesn't explain why they would also purchase less rice.
(E) This is correct. If these families had money to purchase preferable rice substitutes, they may have increased or kept constant their consumption of rice and rice substitutes, but shifted some of their consumption from rice to substitutes because of the coupons.