Which of the structures shown is NOT a resonance contributor of the ion shown? (One of the options repeats the ion exactly as drawn.)

The ion shown is hexa-2,4-dien-1-ylium, a substituted pentadienyl cation. Its contributors place the positive charge on C1, C3 and C5 of the pentadienyl chain, which are the first, second and fourth structures (the fourth being the ion as drawn). The third structure has the same formula but a hydrogen has moved from one carbon to another, so it is an isomer (a vinyl cation), not a resonance form.