Mutations in DNA binding sites for proteins would affect any other genes in the operon, and a plasmid with a wild type binding site would not affect these other genes since the wild type binding site would not be on a physically continuous piece of DNA with those other genes. This is an example of cis-dominance. In contrast, mutations in the binding proteins could be overcome by adding a plasmid with a wild type protein since it moves to the genes and does not have to be physically/covalently linked to them.
Work Step by Step
To understand this, the key is to see which case requires physical continuity: the mutations in the DNA binding site. A plasmid is useless for correcting this error since it will not be physically continuous with the affected genes. The protein in question, however, can move to the DNA, so a wild-type protein can correct the mutation in that case.