Answer: Add 150 ml of water to 100 ml of 2.5 M stock solution.
Step-by-step explanation: According to the neutralization law, 
 
where, 
 = molarity of stock solution = 2.5 M 
 = volume of stock solution = ? 
 = molarity of desired solution = 1.0 M 
 = volume of desired solution = 250 ml 
Now put all the given values in the above law, we get the volume of stock solution. 
 
 
Therefore, 100 ml of 2.5 M of stock solution is taken and 150 ml of water is added to make the volume to 250 ml.