Angles can be congruent based on various theorems.
In this case, the statement that completes the proof is: [tex]\mathbf{\angle AMB \cong \angle DMC}[/tex]
The angles between two transverse lines are congruent.
The above statement means that, the vertical angles at the point of intersection of the transverse lines are congruent
This is so, because the two angles are vertical angles
From the diagram (see attachment), the vertical angles are:
Because both angles are vertical angles, then:
[tex]\mathbf{\angle AMB \cong \angle DMC}[/tex]
Read more about vertical angles at:
https://brainly.com/question/17856245