Option 1 -> Rs.15 is incorrect as it doesn't match the difference between total costs.
Option 2 -> Rs.10 is incorrect; this underestimates the additional cost.
Option 3 -> Rs.20 is the difference between Rs.115 and Rs.95, which represents the marginal cost.
Option 4 -> Rs.25 is incorrect as it overestimates the cost difference.
Hence, Option 3: Rs.20 -> Marginal cost is calculated as the change in total cost when one additional unit is produced. MC = TC₈ - TC₇ = Rs.115 - Rs.95 = Rs.20. This Rs.20 represents the extra cost incurred specifically for producing the 8th unit of output. -> correct