3 pumps, working 8 hours a day, can empty a tank in 2 days, How many hours a day must 4 pumps work to empty the tank in 1 day?

None of these

Let's calculate the total pump-hours required to empty the tank:

3 pumps * 8 hours/day * 2 days = 48 pump-hours

If 4 pumps need to empty the tank in 1 day:

Let x be the number of hours they need to work:

4 pumps * x hours/day * 1 day = 48 pump-hours

4x = 48

x = 12