Question

One gram of water has a volume of 1 millimeter. What is the volume of the water if it has a mass of 1 kilogram

Answer 1

There are 1,000 grams in a kilogram. 
Each gram of water has a volume of 1 milliliter (the question said millimeter but that measures length and I think it was meant to say milliliter). 
Since there are 1,000 grams with a volume of 1 millimeter each, the volume is 1,000 milliliters, which is equal to 1 liter. 
Hope this helps!