Define opportunity cost.

Opportunity cost is the value of the next best alternative that must be foregone when a choice is made.

If the farmer grows wheat, the output is 6 tonnes per hectare.
If the farmer grows barley, the output is 4 tonnes per hectare.

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.

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 $\text{D}$.