If S is sixth, then O can’t be fifth. They can’t be beside each other (rule 6). So O must be first instead, because O can only be first or fifth (rule 3).
I’ve drawn TF and RT under the diagram. Do they look familiar? The first rule says we need either TF or RT. And spaces 4 and 5 are the only place they’ll fit.
C is CORRECT. Either F or T has to go fifth.
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