Calculations regarding Buffer stocks? Here's a question which I don't quite know how to answer. Originally, my answer was (D), though when I checked it the correct answer was (A). As far as my knowledge goes, buffer stocks are buying/selling of stocks by the government to help manipulate supply. Please help provide some elucidations, many thanks.
The answer is indeed A:

Initial farm income is $\$5 \times 2,000 = \$10,000$. 

If the supply is $S_2$, then current income is below that figure. 
However,  if the government takes away from the market 1,000 tonnes,  then the supply curve will shift left by that amount, resulting in a new equilibrium with 4,000tonnes @ \$2. 
Therefore farm income is now:  $\$2 \times 4,000 = \$8,000$. The government buys the buffer at market price,  thus $\$2 \times 1,000 = \$2,000$. We end up with the same farm income as in the initial situation: $\$8,000 + \$2,000 = \$10,000$.