In order to solve this, you need to know the computation that you will use. You only have 70% dextrose of 1000 ML and you are feeling confused with how much of this dextrose should you use. You need to compute 70 x 1000 x the solution that you need.
You can compute this as 0.25 x 1000. Your equation should be( 0.70 x 1000) (X= 0.25 x 1000.) From this, you will see that X = 0.357 which means that you will need =357 ML dextrose. Subtract it with 1000 ML. 1000 - 357 = 643 ML water. Based on the answer, the right choice is letter B.