This is a rather simple dimensional analysis problem. Based on the given information, we can use dimensional analysis to find how many millimeters of sodium chloride solution can be prepared:
Now, we need to consider the amount of significant figures for this problem, which is 1 because both 100 g NaCl and 0.1 mass fraction (as given) both contain one significant figure. In a chemistry problem like this, you look at the given value with the least significant figures (in this case, 1) and this is the amount that your answer will contain.
So your answer would be
I hope that helps!