Answer:
A. Triangle sum theorem.
Step-by-step explanation:
We have been given that [tex]m\angle 1+m\angle 2+m\angle 3=180^{\circ}[/tex].
We know that angle sum property of triangle states that three angles of a triangle add up-to 180 degrees.
Since the sum of angles of a triangle is 180 degrees, therefore, option A is the correct choice.
