Answer - 2/3
If the recipie Liam is using requires 3 cups of flour, and he only has 2, then he has 2/3 of the amount in the recipie. This means he would need to use the same proportions of sugar as flour - 2/3. Therefore he needs to use 2/3 x 1 of the amount in the recipie, which is two thirds (2/3) of a cup.