The problem is asking you to choose an equation that should be written as statement 4 of the proof.
Statement 4 is an algebraic expression of the verbal statement 3. The marked response is correct. It expresses the relationship that is the <em>definition of supplementary angles</em>.
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Often you can gain a clue in these proofs by looking at places that use similar reasons or similar statements. Lines 5 and 6 are the algebraic and verbal representations of the relation between angles 1 and 2. This matches lines 3 and 4, which are the verbal and algebraic representation of the same relationship between angle 1 anf 4.
