The statement "m∠MRS = m∠MRO" is justified by the Vertical Angles Theorem, which states that when two lines intersect, the vertical angles formed by the intersection are congruent. In this case, ∠MRS and ∠MRO are vertical angles formed by the intersection of line RS and line MO, so they must have the same measure. Therefore, m∠MRS = m∠MRO.