If there are /6, then there are six parts of homework to do. If she has 1/6 homework left, that means she already did 5/6 of her homework. 5/6 - 2/6 she did after dinner is 3/6, or the amount of homework she did before dinner. Since three is half of six, we can simplify this expression to 1/2.
So the correct answer would be that Chloe did 1/2 of her homework before dinner.