To fill in the blank in the proof of the Base Angles Theorem, you should use the statement that involves the congruence of triangles based on pairs of sides being congruent.
The appropriate statement to fill in the blank is:
"I now have three pairs of sides congruent, so △ABD≅△CBD by the SAS Congruence Theorem."
This statement correctly follows from the previous information given in the proof and leads to the conclusion that the angles are also congruent (CPCTC).