Rn is the last element of 6th period and has an atomic number of 86. So the last electron enters in the 6p subshell. Thus option (iii) is correct.
Option i) and ii) are incorrect as the last electron enters the 7th shell therefore they belong to 7th period.
In case of option iv) the configuration shows that the element is not the last element as it would have a stable orbital.
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