Fia needs 3/4 cub of sugar
She has only a 1/3 cup and a 1/8 cup
To find which cup should she use :
Divide 3/4 by 1/3 and divide 3/4 by 1/8
The operation which give integer number is the true choice
So,

So, it should be use a 1/8 cup, so 6 times of a 1/8 cup will give 3/4 cup of sugar.
So, the answer is : a 1/8 cup