From the curve above, at what point is it advisable for the farmer to stop further addition of input?

The farmer should stop adding input at point III. Point III represents the point of diminishing returns, where the increase in output from adding more input starts to decrease. In other words, the farmer will get less and less additional output for each unit of input they add after this point. For example, adding more fertilizer may result in a larger increase in crop yield up to a certain point, but beyond that point, adding more fertilizer may not result in much of an increase in yield. It is important for the farmer to recognize this point and stop adding input, as continuing to add input beyond point III will not result in a significant increase in output, but will still increase the cost of production.