Let's plug in some numbers (easy ones!).

let's say a filled box weighs 10lbs total. So 10% of that weight is the empty box (0.1*10lbs=1lb).

So if a partly filled box weighs half of a totally filled box (10lbs):

Totally Fill:
1lb (empty) + 9lbs fill =10lbs

Partly Fill:
1lb (empty) + xlbs = 5lbs (half of 10).

So x = 4.

How much of the box is full?

4lbs out of a possible 9lbs is full.

4/9, hence D.