Question

What else would need to be contruent to show that ABC=DEF by the AAS theorem

Answer 1

The answer is B because if you follow it starting from it AAS
 

Answer 2

The answer is B. ∠C = ∠F.
An AAS congruency of two corresponding triangles has a congruent angle, angle, and side in a row. The order of the angles and sides matters, so in order for the triangles to be congruent by the AAS theorem, ∠C and ∠F must be proven congruent angles. 

Hope this helps! :)