Option B:
Triangle sum theorem allows her to write the statement.
Solution:
Gitta writes m∠1 + m∠2 + m∠3 = 180°
To find which reason allows her to write this statement:
∠1, ∠2 and ∠3 are interior angles of the triangle.
Triangle sum theorem:
Sum of all the interior angles of the triangle is 180°.
⇒ m∠1 + m∠2 + m∠3 = 180°
Hence triangle sum theorem allows her to write the above statement.
Option B is the correct answer.
