Answer:
A. 4/3 = DE/9
Explanation:
Because the triangles are similar, we can use a proportion of each triangle's corresponding side lengths to find their measures.
DE corresponds to AB
DF corresponds to AC
EF corresponds to BC
Now, we can set up a proportion with any set of corresponding side lengths to find the measure of DE (a proportion is set up correctly if corresponding side lengths are at the same location in the proportion: both in the numerator or both in the denominator).
As "A" is the only choice that sets up the proportion correctly, it is the answer.