Rearrange the parts of the sentence in correct order.
It may not be a simple
P-someone does stand to gain
Q-by keeping farmers divided
R-case of "divide and rule", but

* The sentence opens with "It may not be a simple...". The noun phrase must be completed, which naturally connects with R ("case of 'divide and rule', but").
The conjunction "but" in R sets up a contrast clause, which is introduced by P ("someone does stand to gain").

Q completes the action of gaining ("by keeping farmers divided").
Therefore, the correct order is RPQ.