Your answer would be D because you ant make a triangle out of these measurements. <span />
Let the number of water pumps be n and the time taken to fill the pool is t.
Now, we can see that higher the number of water pumps, lesser the time to fill the pool.
It means quantities vary indirectly.
Thus, 
Now, we have been given that It takes four identical water pumps 6 hours to fill a pool.
Thus, we have

Hence, constant of variation is 24.
Now, we have to find the time that three pumps will take to fill the same pool.

Hence, it will take 8 hours to fill the pool by three identical pumps.
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