Because of high prices on grain crops in Mordovia, fields us

.

IMO C

CONCLUSION::next year total profits of Mordovian farmers will increase.
PREMISE 1 :next year most Mordovian farmers plan to markedly increase the proportion of their arable lands used for growing grain crops.
PREMISE 2: fields used for growing those crops bring farmers significantly more profits per acre of land than fields used for growing other crops.
means :INCREASE IN ARABLE LAND ===>INCREASE IN GRAIN===>INCREASE IN PROFIT
CLEARLY this will break if INCREASE IN CROP will lead to DECREASE IN PRICE.
So in short it is assuming that increase in crop will not lead to decrease in price
or in other words
Prices on grain crops are high in Mordovia for reasons other than short supply of those crops.

HENCE C is the assumption.

(A)The proportion of imported grain crops sold in Mordovia is very small.==>this will not affect even if proportion increases because still the per acre will provide more profit.WRONG
(B)Next year, prices on crops grown in Mordovia other than grain crops will not increase.
We are not concerned about other crops ..we need to concentrate only on our CONLUSION that next year total profits of Mordovian farmers will increase. WRONG
(C)Prices on grain crops are high in Mordovia for reasons other than short supply of those crops.
correct
(D)Most farmers plan to increase the proportion of their fields used for growing grain crops in order to gain greater profits.
if they are growing with intention or not with intention of gaining profit this will not affect conclusion anyhow.WRONG
(E)Mordovian farmers who export grain crops generally earn more than farmers who grow grain crops for domestic market.
OUT OF SCOPE.WRONG

waiting OA