What additional information would you need to prove that δabc ≅ δdef by sss?
1.ac=df
2.ac=fd
3.af=cd
4.fa=cd

For the answer to the question above asking What additional information would you need to prove that ΔABC ≅ ΔDEF by SSS? 
The answer to your question is AB = DE & BC = EF, so the only sides missing are AC = DF, 

so Side AC is congruent to side DF is the answer.

I hope my answer helped you. Have a nice day!