Option 1 -> Rs. 80 is the final total cost, not the marginal cost.
Option 2 -> Rs. 13 is the change in total cost when production increases by 1 unit, which is the marginal cost.
Option 3 -> Rs. 536 does not correspond to any relevant calculation in this problem.
Option 4 -> Rs. 720 does not correspond to any relevant calculation in this problem.
Hence, Option 2: Rs. 13 -> Marginal Cost (MC) is calculated as the change in total cost divided by the change in quantity. Here, MC = (80 - 67) / (9 - 8) = 13 / 1 = Rs. 13. This represents the additional cost incurred to produce one more unit of cotton. -> correct