I keep getting the same wrong answer..

I think anwer is G. 6 units long

$AD=AB + BD$

so  $AB = 30 - 20 =10$

and $AB = AC + CD$

so $CD = 30 - 16 = 14$

Finally $AD = AB + BC + CD$

so $BC = 30 - 10 - 14 = 6$

so $BC = 6$

AD =30, AC =16

AD - AC =30 -16 =14 - the length of CD

But BD = 20, so:

BD - CD =BC

20 - 14 = 6 - the length of BC!!, or "G".

[AD  -  AC]  =  [30  - 16 ] = 14  = CD

[BD  - CD ]  = [20  - 14]  = 6   =   BC