What else would need to be contruent to show that abc=def by the aas theorem

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. 

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

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