The rainfall on five successive days were measured as 100 mm, 80 mm, 60 mm, 40 mm and 20 mm respectively. If the infiltration index or the storm loss rate for the catchment area is earlier estimated as 50 mm/day, the total surface run off will be A. 50 mm B. 60 mm C. 90 mm D. 140 mm

The correct answer is $\boxed{\text{C) 90 mm}}$.

The total rainfall for the five days is $100 + 80 + 60 + 40 + 20 = 300$ mm. The infiltration index is the rate at which water is absorbed into the ground. In this case, the infiltration index is 50 mm/day. This means that for each day, 50 mm of the rainfall will be absorbed into the ground. The total surface runoff is the amount of rainfall that is not absorbed into the ground. In this case, the total surface runoff is $300 – 50 \times 5 = 90$ mm.

Option A is incorrect because it is the infiltration index. Option B is incorrect because it is the amount of rainfall on the first day. Option D is incorrect because it is the total rainfall.