Answer: Angle Addition Postulate
Explanation:
Angle Addition Postulate says that if we put two angles one after another together forms a new angle that has measure equals the sum of the measures of the two actual angles.
In the given proof ,the statement in box 1 is
m∠UVX=m∠UVW+m∠WVX
Here, ∠UVX= is a new angle formed by adding ∠UVW and ∠WVX.
They are sharing the same vertex V.
So the correct reason for this statement is "Angle Addition Postulate".