Answer:
D. 6 feet.
Explanation:
The third longest side of ABCD is 12 feet long and the third longest side of EFGH is 18 feet long.
As they are similar the corresponding sides are in the same ratio so
12/18 = x/9 where x is the 4th side of ABCD.
2/3 = x/9
3x = 18
x = 6 feet.