The missing reason in the proof is:
Transitive Property of Equality
This is the reasoning that connects statements 1, 2, and 3. Since \( m∠A + m∠C = 180° \) (statement 1) and \( m∠A = m∠E \) (statement 2), it follows by the transitive property that \( m∠E + m∠C = 180° \) (statement 3).