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 of the storm loss rate for the catchment area is earlier estimated as 50 mm/day, the total surface run off will be:

Correct option is C

Surface runoff is the amount of water which contributes into effective rainfall after infiltration losses.
Total surface runoff = (100 -50) + (80-50) + (60-50) + (40 - 50) + (20 - 50)
= 50 + 30 + 10 + 0 + 0 = 90 mm
Note if (rainfall - infiltration loss) < 0, that take it to be 0.