In order to prove that the angles 1 and 3 are congruent through the congruent supplements theorem, we need to show that the angles 1 and 3 are supplementary to the same angle.
On the 3rd statement, the angle 3 and the angle 2 are identified as supplementary angles.
Then, the missing statement would be to identify the angles 1 and 2 as supplementary angles by the linear pair theorem.
Therefore, the correct choice is option A) 1 is supplementary to 2, linear pair theorem.