。
She mistook the sugar for salt.
Literal
She [topic-は] sugar [object-を] salt [as-と] mistook.
間違える with the structure [X]を[Y]と間違える ('mistake X for Y') uses と as a marker of the comparison — の identification — 'took X to be Y.' The English 'mistake X for Y' has the same word order. Japanese also has 間違う ('to be mistaken'), the intransitive counterpart, but for transitive 'mistake X for Y' you reach for 間違える. A small kitchen mishap with a long literary tradition: 砂糖と塩 sit together in identical white containers in many Japanese kitchens, making this confusion a domestic universal.