Examine the paragraph proof. Which theorem does it offer proof for?

lines WZ and XY intersect at point V

Prove: ∠XVZ ≅ ∠WVY

We are given an image of line WZ and line XY, which intersect at point V. m∠XVZ + m∠ZVY = 180° by the Definition of Supplementary Angles. m∠ZVY + m∠WVY = 180° by the Definition of Supplementary Angles. Since the sum of m∠XVZ + m∠ZVY = m∠ZVY + m∠WVY by the Transitive Property of Equality, m∠ZVY can be subtracted from both sides of the equation because of the Subtraction Property of Equality. Therefore, m∠XVZ = m∠WVY and ∠XVZ ≅ ∠WVY by the definition of congruent angles.

Alternate Interior Angles Theorem
Corresponding Angles Theorem
Vertical Angles Theorem
Same-Side Interior Angles Theorem