When multiple objects are performing work at the same time, you add their rates to find the total rate. So if four pumps are filling a tank, their total rate is the sum of all their individual rates.
Let's set the big pump equal to rate p.
So the rate of each small pump = (2/3)p
The sum of all four pumps is p + (2/3)p + (2/3)p + (2/3)p = 3p
So together, the four pumps work 3 times as fast as the original pump. The original equation in w = rt format is W = (p)t, where p is just the big pump.
In order to make the equation balance with our new rate (3p), we must adjust the time as follows: W = (3p)(t/3). The 3s cancel and we have our original equation W = (p)t. So the new time is 1/3 the original time required. B is the answer.
Hope this helps!