6m cube can be converted to cm cube, that would be 6x100x100x100 cm^3 = 60,00,000 cm^3
the question says "choose the rates at which the tank would be emptied 1/3rd of the initial capacity which means we need to select rates at which the the water would be emptied more than 60,00,000/2 = 20,00,000 cm^3

in 2 days there are 172800 seconds, and if the outlets empty the water at a constant rate

a) would empty 172800x16= 27,64,800 cm^3 of the tank
b) would empty 172800x10 = 172800 cm^3 of the tank ( this way you can rule out 9 and 9.5 from the option)
c)would empty 172800x12 = 20,73,600 of the tank ( d can be included in the option since outlet c having lesser rate than d empties more than 1/3rd of the tank)

There's absolutely no need to solve each and every bit. you need to eliminate the options based on minimal common sense. this would save a lot of time on the main exam .

Regards,
sumi