Answer: 2
Steps: You can find the sides using the proportional way. As in the question it is already said the triangles are similar so they have one value that can be added/subtracted/divided/multipled in order to find either of a length. The bigger triangle, if you divide each side by 4, the answer comes.
16/4 = 4 or AB/4 = DE
12/4 = 3 or CB/4= EF
8/4 = 2 or AC/4= DF
So 2 is the length of side DF.