Answer: The last option.
Explanation:
The statement that can be used to prove that vertical angles ∠1 and ∠3 are congruent is;
m∠1 + m∠2 = m∠2 + m∠3
This is because we know that m∠1 + m∠2 is a straight line and that m∠2 + m∠3 is a straight line. Straight lines are equal to 180 degrees, so the vertical angles must be congruent.