Triangles are similar, so:
[tex]\dfrac{|AB|}{|DE|}=\dfrac{|AC|}{|DF|}\\\\\\\dfrac{2,5}{1,5}=\dfrac{4}{|DF|}\\\\\\
2,5\cdot|DF|=1,5\cdot4\quad|:2,5\\\\\\|DF|=\dfrac{1,5\cdot4}{2,5}=\dfrac{6}{2,5}=\dfrac{6}{2,5}\cdot\dfrac{4}{4}=\dfrac{24}{10}=2\dfrac{4}{10}=\boxed{2,4\,u\qquad[\text{B}]}[/tex]
