Which sentence shows a parallel structure error?

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Parallel structure means that items in a list should share the same grammatical form. In a list of actions, that usually means all endings should match.

In the sentence with hiking, to swim, and biking, the first item uses a gerund (hiking), the second uses an infinitive (to swim), and the third uses another gerund (biking). That mix breaks the pattern, so the list isn’t parallel. The other sentences keep the same form throughout the list (all -ing forms in two of them, or a consistent infinitive form in another), which is why they don’t show the same error.