To prove a similarity of a triangle, we use angles or sides.
In this case we use angles to prove
∠ACB = ∠AED (Corresponding ∠s)
∠AED = ∠FDE (Alternate ∠s)
∠ABC = ∠ADE (Corresponding ∠s)
∠ADE = ∠FED (Alternate ∠s)
∠BAC = ∠EFD (sum of ∠s in a triangle)
Now we know the similarity in the triangles.
But it is necessary to write the similar triangle according to how the question ask.
The question asks " ∆ABC is similar to ∆____. " So we find ∠ABC in the prove.
∠ABC corressponds to ∠FED as stated above.
∴ ∆ABC is similar to ∆FED
Similarly, if the question asks " ∆ACB is similar to ∆____. "
We answer as ∆ACB is similar to ∆FDE.
Answer is ∆ABC is similar to ∆FED.