Define opportunity cost.

D. 6 tonnes of wheat Step 1: Define opportunity cost. Opportunity cost is the value of the next best alternative that must be foregone when a choice is made. Step 2: Identify the options and their outputs. If the farmer grows wheat, the output is 6 tonnes per hectare. If the farmer grows barley, the output is 4 tonnes per hectare. Step 3: Determine what is given up. The farmer decides to grow barley instead of wheat. This means the farmer chooses to produce 4 tonnes of barley. By making this choice, the farmer gives up the opportunity to produce 6 tonnes of wheat on that same hectare of land. Step 4: State the opportunity cost. The opportunity cost of growing barley is the amount of wheat that could have been produced instead. Therefore, the opportunity cost is 6 tonnes of wheat. The final answer is $\boxed{\text{D}}$. That's 2 down. 3 left today — send the next one.