Answer:
A
Explanation:
AC = 5
DF = 15
5 x 3 = 15
BC = 3
EF = 9
3 x 3 = 9
Do you see the pattern? So as you can see the difference between Triangle ABC and DEF is the multiply/divide 3 so if you want to find DE then you should set it up as:
 =
 = 
 and to check if its right if you solve it you get
 and to check if its right if you solve it you get 
 =
 = 

and as you see 
 and
 and 
 are literally the same thing just you need to simplify
 are literally the same thing just you need to simplify 
